EoW 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. 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. It features a low modulus, very low T g

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.

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.

Figure 1 ▲ ▲ : Microbend sensitivity versus primary modulus for 50µ multimode fibre

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EuroWire – November 2008