This paper reports on the first steps taken by TeliaSonera towards deploying optical performance monitoring (OPM) in the company’s transport network, in order to assure increasingly reliable communications on the physical layer. The big leap, a world-wide deployment of OPM still awaits a breakthrough. There is required very obvious benefits from using OPM in order to change this stalemate. Reasons may be the anaemic economy of many telecom operators, shareholders’ pushing for short-term payback, and reluctance to add complexity and to integrate a system management. Technically, legacy digital systems do already have a proven ability of monitoring, so adding OPM to the dense wavelength division multiplexed (DWDM) systems in operation should be judged with care. Duly installed, today’s DWDM systems do their job well, owing to rigorous rules for link design and a prosperous power budget, a power management inherent to the system, and a reliable supplier’s support. So what may bring this stalemate to an end? -A growing number of appliances of OPM, for enhancing network operation and maintenance, and enabling new customer services, will most certainly bring momentum to a change. The first employment of OPM in TeliaSonera’s network is launched this year, 2004. The preparedness of future OPM dependent services and transport technologies will thereby be granted.
In this paper the reliability is calculated of an all- optical C-band, 45-carrier WDM ring network comprising a large number of gain-flattened EDFA amplifiers and 10 add/drop nodes with full connectivity. A ring of the assumed type and size would cover a region surrounding the Baltic Sea, forming the top layer of a hierarchical network. Protection switching is technically uncomplicated but expensive, and may cause transmission problems due to the long distances involved. In order to estimate the reliability of the ring, the FIT number for longest optical paths is calculated and compared with requirements according to ETSI standard, a priori assuming that neither diversity, nor protection switching is available. This is obviously true for a single fiber ring in which all aggregated traffic is unidirectionally routed. From the architecture chosen follows that certain components will be crucial for the network's performance. In this context, pairs of co-working pump lasers may provide a sort of redundancy which would be a useful means to improve the reliability of the network. With this in mind, the network's resilience to non- catastrophic component failures has been simulated, with a focus on pump- and signal lasers. The global effects on optical SNR and power due to single- and multiple failures of these components are studied for arbitrary local combinations. The study enlightens a network mechanism for spectral distortion caused by pump failure, and an efficient remedy to significantly restore the signal quality.
The reliability of long lengths of fibers in optical cables is determined by the presence of weak spots which are infrequently produced by normal fiber production methods. Particles from the atmosphere may cause weak spots on the fiber surface during the drawing process. Abrasive damage of the fiber surface may occur when physically contacting the drawing equipment. Finally, interior weakening of the fiber may occur during preform manufacture because of voids and particles, embedded inside the preform. Four representative types of such flaws have been studied with respect to their mechanical fatigue in normal atmosphere, 23 degree(s)C, and 50% relative humidity. The results are summarized in this paper. Fatigue of zirconia induced weak spots was observed in rapid tests, being significantly less or absent, at lower stress rates. No fatigue was observed in fiber with particles of chromium oxide deeply embedded in the glass. The effect of the size of particles was also studied, by comparing tensile test results with a theoretical strength distribution, based on an assumed relation between particle size and fiber strength. The size of particles appeared to have no significant influence on the atmospheric fatigue of fiber. A significant time-dependence of strength was found in weak spots caused by physical contact with glass during drawing. The static fatigue is apparently the same as for flaw-free fiber. No fatigue was found of weak spots originating from abrasive contact with metal, in the coating die.
A radically different mechanical test, the `distributed strain test,' has been studied theoretically, showing promising results. Owing to the range of loads applied to the fiber, and the convenient data acquisition, the procedure of measurement and evaluation of the fatigue of fiber may be extremely simplified compared to regular tests.
During the past years several actions have been taken by Swedish Telecom in order to ascertain the reliability of optical fiber networks. The status of some current and future works are briefly described in the paper, focusing on the characterization of optical fibers.
More than 10 existing lifetime models for optical fibers, based on a power law to describe the stress-induced crack growth, are studied and compared. The number of models is reduced to only 1 basic model. This model is taking into account the effects of proof test. An alternative model, to be used when proof testing is performed on-line, is given as a worst-case limit of the basic model. A choice of 3 testing methods to obtain information about the weak-flaw distribution is given: dynamic-fatigue and variable screen-testing of long lengths or using the failure number during proof test. The models can also be used for titanium-doped fibers, but then a correction must be made for the compressive surface-stress. The lifetime models cannot be used for carbon-coated fibers and for fibers in water without further study.
New approaches to improve the prediction of failure of long length fibers are presented. They are currently being used in type approval and quality control of optical fibers and cables in Swedish Telecom. One approach is to use the B-parameter. Values on this parameter, usually obtained from testing in liquid nitrogen, high temperature, or under vacuum, are not being considered very useful for lifetime estimation in regular environments. Therefore, a high- speed tensile test has been developed, by which the B-parameter is evaluated, at adequate service conditions. The other approach, to long-term fatigue data, is the expander test by which static and dynamic tests are made on medium length fibers under uniaxial stress. Long- term failure statistics are conveniently accessed by testing a single specimen. Limitations of the techniques are discussed: influence on fatigue parameters of static versus dynamic load and testing environment, and fatigue of high strength versus low strength modes.
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