About Optical Fiber Reliability

Optical fiber is in widespread use for a variety of applications. Mechanical reliability is an important issue since fracture of the fiber results in a loss of optical transmission. The strength of fiber is a stochastic variable because it is controlled by the presence of flaws from several sources and with a wide range of severity. It is therefore necessary to make many strength measurements in order to characterize both the mean of the strength distribution as well as its width. An additional result of the stochastic nature of strength is that the strength of a specimen depends on its size – larger specimens are more likely to contain a large flaw and are therefore, on average, weaker than smaller specimens.

In addition, strength is time dependent. This is because atmospheric moisture attacks the flaws resulting is a slow degradation with time, often called "fatigue". Therefore, long-term strength is not immediately related to the strength measured on short time scales in the laboratory. To characterize this effect, it is necessary to make strength measurements over a range of loading rates in order to extrapolate the behavior to fatigue under service conditions. It should be noted that polymer buffer coatings do not exclude moisture from the fiber surface – moisture can permeate through the coating on the time scale of minutes to hours.

Fiber Sigma manufactures a range of test equipment specifically designed to measure the strength and fatigue behavior of optical fibers. The thin, flexible geometry of an optical fiber means that special techniques are required for gripping the fibers. Our equipment solves these gripping problems.

Tensile Testing

Strength measurement techniques for optical fibers fall into two general categories. The first is tensile testing in which the ends of the fiber are pulled in simple tension. We provide a Table Top Tensile Tester that can test lengths of fibers in the range of a few cm to a meter. For longer lengths, an option for the Table Top Tensile Tester is available that is capable testing fibers of up to 20meters gauge length or even more. These systems are much less expensive that a typical load frame style tensile tester and are more versatile since the fiber can be immersed in liquid test environments as well as in ambient air.

Bend Testing

The second approach to measuring strength is to use flexural or bending techniques. Experimentally, these techniques are more convenient to perform than tensile testing but test only a short length of fiber. Bending techniques are very useful for characterizing the effect of different polymer coatings and test environments on strength and fatigue. A broad range of test environments can be used – the small bent section of fiber can be immersed in a beaker of liquid or a gaseous test environment can be blown over the fiber, thus obviating the need for control of temperature and humidity of ambient laboratory air.

Fiber Sigma produces the Two-Point Bend System that breaks fibers in two-point flexure – so called because the fiber is bent double between two faceplates and makes contact at two points of contact. This technique can be used for loading under a very large range of loading rates and can also test up to 30 fibers simultaneously. A recent addition to our product line is the Automatic Fiber Loader which loads specimens into the Two-Point Bend System and then removes the broken ends, all without operator intervention. The equipment can be started and then left to run a complete fatigue experiment (measuring the strength of many specimens for many different loading rates) without help. This improves efficiency and reduces operating costs of the equipment.

Two point bend testing has recently gained more widespread acceptance as it is the only option for testing when using fibers with a very weka primary coating that tears apart when testing using a tensile tester.

Two-point bending is most useful for testing comparatively strong fibers. Very weak fibers can be tested using our Four-Point Bend Apparatus. While this equipment is more of a research tool than a production or quality assurance tool, we supply it to complement our other products.

Static and Dynamic Fatigue Testing

The fatigue behavior can be characterized by either static or dynamic fatigue testing. Dynamic fatigue involves measuring the strength of the fibers as a function of the loading rate. All the equipment described above can perform this type of testing. Alternatively, static fatigue involves applying a constant (i.e. static) stress to the specimen and then measuring the time to failure. All our strength measurement equipment can be run in static mode but this is not usually the most effective way to perform static fatigue. The most useful aspect of static fatigue is that it can be used for very long duration testing of fibers (for years or longer). We produce apparatus for performing long term static testing in two-point bending. Many fiber specimens can be inserted into precision diameter tubes and their failure can be monitored acoustically. Acoustic detection equipment, the Four Channel Trigger Box, is available together with precision bore tubes. The Four Channel Trigger Box monitors up to four separate channels and produces electrical signals for interfacing to a chart recorder or computer. Our Trigger Box Monitor System is a PC based data acquisition system that monitors up to 16 Trigger Boxes, i.e. up to 64 separate channels of fatigue data. Essentially this system gives statistical results out from acoustic pulses in. The statistical analysis of data is detailed, including the ability to predict the results of experiments currently in progress using analysis of censored data.

Data Analysis and Graphing

Strength and fatigue data need to be analyzed in order to extract statistical and fatigue parameters. Data Analysis Plug-ins for the SigmaPlot program are available which produce publication quality graphs and data analysis compliant with current standard test procedures.

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