EoW January 2013

53 / 88

Technical article
January 2013 51
Test methods for cables
incorporating reduced bend
radius fibres
By Wayne Kachmar, ADC Telecommunications, USA
Abstract
This article will attempt to compare
mechanical performance parameters of
waveguide cables with optical perfor-
mance 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 pre-
scribed by standardised tests such as
Telcordia GR-409 and GR20.
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 conventional 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
▼ ▼ Figure 2: Fibre with >4.5% shrinkage
▼ ▼ Figure 3: <5% shrinkage cable connector interface
3.0mm Jacket
3.0mm Jacket
900 Micron Tight Buffer
900 Micron Tight Buffer