WCA March 2013

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. Macrobends and Microbends 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 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. In conventional 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. Insufficient test methods 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 ❍ ❍ Figure 5 : Optical Fibre Strain Gauge Measurement System

❍ ❍ Figure 4 : FOTP-33 Long Gauge Tensile Test Fixture

53

www.read-wca.com

Wire & Cable ASIA – September/October 2007 Wir & Cable ASIA – March/April 13