WCA March 2013

Test methods for cables incorporating reduced bend radius fibres By Wayne Kachmar, ADC Telecommunications, USA

Abstract This article will attempt to compare mechanical perfor- mance parameters of waveguide cables with optical performance of both conventional fibres and reduced bend radius fibres. The coordination of mechanical and optical test data can point to more appropriate test criteria for cables with reduced bend radius fibres. This will ensure a more robust characterisation criterion appropriate to this new class of fibre. Introduction The advent of both single mode and multi-mode bend insensitive fibres has brought into question whether existing cable test plans accurately characterise a cable design to provide expected lifetimes. Presently, most published test plans rely on delta attenuation values at discrete wavelengths as pass/ fail criteria for various mechanical criteria. With the introduction and use of new reduced bend radius fibre types, less robust cable designs can now pass such standardised cable tests. This result can lead to possible substandard cable designs that can generate future failures in the field. Long-term stresses may be placed on the optical waveguides and not be reflected in the delta attenuation measurement protocols currently prescribed by standardised tests such as Telcordia GR-409 and GR-20. Fibre improvements Telcordia GR-409 is the current standard of specifications for indoor fibre cables, while Telcordia GR-20 provides technical reference for outdoor cables. Some companies, such as Verizon, have more sophisticated versions that reference GR-20 and GR-409, but also add additional qualifications. Together, this specification documentation dictates the mechanical performance standards agreed upon by the customer and manufacturer. More recently, however, fibre improvements, particularly in reduced bend radius fibres, are challenging the industry to revisit the test standards for fibre.

With the increased performance features of reduced bend radius fibres versus conventional fibres, existing standards may no longer be a “one size fits all” measurement. Several optical fibre manufacturers developed con- ventional optical fibres in the 1970s. Over the years, there have been few significant improvements outside of coating developments to improve the fibre’s inherent ability to withstand mechanical forces on its environment. But aside from innovations during the draw process, improving the overall empirical tensile properties of optical waveguides, improvements to optical fibre designs have been relatively minor until about five years ago. At that time, several concepts emerged to improve on other fibre characteristics, such as physical strength and bending characteristics. This was the introduction of reduced bend radius fibres. Reduced bend radius fibres include several viable technologies. ❍ ❍ Figure 1 : Fibre with <1% shrinkage

3.0mm Jacket

900 Micron Tight Buffer

❍ ❍ Figure 2 : Fibre with >4.5% shrinkage

3.0mm Jacket

900 Micron Tight Buffer

❍ ❍ Figure 3 : <5% shrinkage cable connector interface

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Wire & Cable ASIA – September/October 2007 March/April 2013