EoW November 2008

EuroWire – November 2008

technical article

New optical fibre coating

system optimised for

FTTx applications

By Bob J Overton, Draka Comteq, Claremont, North Carolina, USA; and Xavier Meersseman, Draka Comteq, Billy Berclau, France

Abstract

Fibre to the premises/business/home, or

FTTx, brings broadband data transfer

technology to the individual end-user and

is enjoying an accelerating deployment

worldwide.

In this paper, the authors present the

key performance characteristics of a

new coating system designed for FTTx

applications where conventionally robust

cable designs are not practical.

The coating system, which may be mated

with small-radius bend insensitive fibre

as well as G.652 and other designs,

provides additional protection against

stress-induced micro-bending.

It features a low modulus, very low T

primary coating for added cushioning

against lateral and axial stresses induced by

external contacts or by low temperature,

and new enhanced colour pigment built

into the secondary for improved brightness

and visibility without inks.

1 Introduction

Fibre-To-The-x installations are making use

of innovative, reduced cost system designs

to facilitate the spread of the technology.

To illustrate, fibre may be delivered in the

last link or links in a form of, for example,

a microcable

[1], [2], [3]

Air-blown fibres provide another efficient

model for delivering the link to the

end-use terminus

[4]

. There continues to

be an industry-wide focus on modes of

deployment that overcome the economic

obstacles in the way of fibre-based

broadband solutions for data transmission

to the business and to the home.

Proposals for various methodologies are

manifold and well known to the reader.

A key deliverable for a successful FTTx

system is a low cost. Reduced size for

cables, drops and structures for blowing

are often critical as well, since putting

through conduits for traditional cable

designs is often prohibitive in existing

infrastructure. Small ducts or tight

pathways already present have to be

usable for new fibre installations. Low-cost

and smallest possible size requirements

drive towards minimising protection

for the optical fibres, that is, away from

conventionally robust, more bulky cable

designs.

Glass designs are now available that offer a

reduced sensitivity to small bending radius,

such as a trench-assisted core design

[5]

or hole-assisted fibres. Glass designs

with lower mode field diameter are less

sensitive to micro-bending stresses, but

are not compatible with G.652 SMF.

Additional protection against micro-

bending is needed to help ensure

successful deployment in all applications

for FTTx. To this end a new coating system

is introduced that is optimised for FTTx,

with the extra demands FTTx places on

fibre and cable structures.

2 Coating design

In developing high quality multimode

coatings, one of the lessons learned is the

benefits of reducing the modulus of the

primary coating.

Figure 1

shows an observed relationship

between the on-fibre modulus of primary

coatings and the micro-bending sensitivity

of the optical fibre. The fibres in this study

are 50μ graded index multi-mode. The

primary coating modulus is characterised

by a method of measuring in-situ, cured

on the fibre

[6]

The micro-bending sensitivity is obtained

using the fixed diameter sandpaper drum

procedure

[7]

. While the lower modulus of

the primary coating can be achieved by

under-curing on fibre, it is desired to tailor

the coating to reach a lower modulus at

nearly full cure. The target modulus is

0.3 to 0.4 MPa for minimising the bend

sensitivity.

A lower modulus for the primary coating

implies a lower crosslink density and thus

a lower concentration of the reactive

acrylate groups.

Figure 1

▲

Microbend sensitivity versus primary modulus for 50µ multimode fibre