EoW January 2013

Technical article

fibre that can cause extrinsic attenuation, a reduction of optical power in the glass. Microbends are defined as nearly invisible imperfections in the optical fibre, usually created during the manufacturing process. These tiny imperfections can also cause a reduction in optical power, or increased attenuation. However, microbends may also occur from the stress compression of the plastics placed on the glass due to polymer shrinkage on the fibre. attenuation increases indicate when a microbend has occurred in the fibre. However, in a reduced bend radius fibre, attenuation changes are typically minimal and the same microbend may not be discovered until an extreme failure in the performance of the cable. Therefore, the failure is going to occur over time as the cable is handled, installed or ages. Modern aging techniques used for testing, such as extreme heat exposure, may not exhibit a failure on today’s new reduced bend radius fibres. The existing test methods for conventional optical fibre are based on mechanical testing and attenuation changes, but they do not specify the cable design being tested. Therefore, if a reduced bend radius fibre is undergoing the same tests, its minimal sensitivity to microbending may allow it to pass the test while a microbend could still cause the fibre to stress over time. That means some cable designs could still be created with inherent failures in design, yet they could pass existing testing standards based solely on what is contained in GR-409 for tight-buffered fibres. In loose-tube outdoor fibre cables, covered by the GR-20 standard, there are a number of tests that may determine whether the fibres are under some stress or strain. Currently, the only requirement for strain testing is contained in TIA-455-33B section FOTP-33a. This covers tensile testing for these cables using a component for measuring fibre strain. The question becomes whether less than five per cent shrinkage, as stated in this specification, is still an acceptable standard or benchmark. It could be too broad a measurement based on the fact that new bend insensitive fibres will not show the same sensitivity. If any flaw or defect in the fibre could possibly be missed by current testing standards, yet could have a significant impact in deployed fibres over time, then new criteria such as fibre strain In conventional fibre, Insufficient test methods

They include ‘trench-assisted’ varieties, ‘voids-assisted’ fibre, photonic-crystal or ‘holey fibres,’ and several other types and technology combinations. When compared with conventional fibre, each of these new innovations has improved the characteristics and mechanical performance of today’s optical fibre. However, during the same time frame, the existing test regimes have remained basically unchanged, continuing to rely on attenuation change based on physical, mechanical and environmental testing. Attenuation continues to be the preferred methodology for determining a fibre’s performance. However, testing reduced bend radius fibres using the same methods for conventional single mode and multi-mode fibre does not take into consideration the unique properties of these new fibres. With that in mind, let’s look at how attenuation is induced in conventional fibres and reduced bend radius fibres. So what exactly changed with the introduction of reduced bend radius fibres? The most obvious improvement was the fibre’s ability to bend more tightly, that is, its bend sensitivity was reduced. These fibres can be bent to a 10, 7.5 or even 5mm radius with no noticeable increase in attenuation or damage to the glass in a long-term environment. Resistance to macrobend and microbend loss was also significantly increased. In fibre optic transmissions, a macrobend refers to a large visible bend in the optical Macrobends and Microbends

should be added to current test methods, specifications and standards. What might work in bulk cable may not work in cable connector interfaces, and what may pass testing today might not work over the expected life of the fibre. The existing aging cycle was developed using high temperature only to detect changes in the jacket and buffering compounds, such as hardening, cracking or shrinkage over the aging process. Today, it may be wise to consider whether those compounds will fail or not when testing is based on different parameters. One such area is thermal coefficient of linear expansion. This is the rate of expansion and contraction of a material over a given temperature profile. The rate of polymer change is typically an order of magnitude compared to glass. For example, if continuous shrinkage occurs beyond the normal shrinkage tests and is identified by increased attenuation, how do you detect it in reduced bend radius fibres where no or minimal increased attenuation is detected? The answer is that you would not – until perhaps the fibre reaches a pivot point where it is no longer a viable long-term communications medium. In the loose-tube cable environment, the opposite can potentially occur. That is, there could be too much excess fibre length and the fibre would bunch up – not due to shrinkage, but because an attenuation increase was not detected in the reduced bend radius fibre. The individual tube is not tested for shrinkage separately but may be coiled for several metres in a transition housing and not have the design of the overall cable to control shrinkage in the individual loose tube. The bottom line is that since attenuation resistance is increased in reduced bend radius fibres, microbends and other stresses on the fibre may not be detectable with today’s testing ▲ ▲ Figure 5 : Optical fibre strain gauge measurement system

▼ ▼ Figure 4 : FOTP-33 long gauge tensile test fixture

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January 2013