Optical waveguides – Optical transmission cable
Patent
1994-04-01
1996-09-10
Lee, John D.
Optical waveguides
Optical transmission cable
385123, 385128, G02B 652
Patent
active
055553354
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to coated optical fibres and methods for their production. In particular the invention relates to coated optical fibres which are especially suitable for use in blown fibre systems.
2. Related Art
Optical fibres are now widely used in place of electrical conductors in the communications field. Typically, glass optical fibres have an external diameter in the range 100-150 .mu.m, usually 125 .mu.m. Polymer fibres are normally somewhat larger in diameter. Unlike conventional electrical conductors, optical fibres are generally fragile and easily damaged to the detriment of their performance and lifetime. Consequently, it is important to protect the fibres from damage.
The first step in the protection of optical fibres occurs, at least in the case of glass fibres, immediately after the fibres are drawn and involves the application of one or two layers of synthetic resin coating. This protection, whether one or two layers, is somewhat loosely referred to as the "primary coating", and fibres so coated are sometimes known as "primary coated fibres". Alternatively, and more accurately, the coatings are sometimes referred to as the primary and secondary coatings, and this convention is adopted in this specification. The first coating, which is typically a low-modulus silicone or acrylate polymer is applied to the fibre surface at a point no more than about a meter from the point where the fibre is drawn down. Commonly, the primary coating is UV curable. The primary coating is also known as the buffer layer, since it serves to buffer the fibre from lateral pressure. The reason for applying the primary coating practically as soon as the fibre is formed is that the strength of glass and other small fibres depends critically on the extent to which their surface is free from cracks and microcracks. In order to avoid the formation of microcracks it is important to protect the fibre surface from dust and other causes of abrasion, and to this end the zone between the point of fibre drawing and the point of application of the primary coating is kept short and dust-free. The mechanical properties of primary coating materials are critical to the performance of optical fibres. In particular, the coating should not induce microbends in the fibre and the mechanical properties should be compatible with those of the fibre.
A particularly important consideration is the material's coefficient of thermal expansion (TOE). The difference in TCEs between the material of the fibre (normally a silica-based glass, which means a low TCE) and that of the primary coating (normally with a TCE an order of magnitude or more greater) means that at low temperatures fibres may be subject to considerable compressive stress, significantly increasing optical loss. This effect is generally made worse by increasing the primary coating thickness, and of course with reduced temperatures.
The secondary coating is typically a hard and robust material, such as nylon, to protect the primary coating, and hence the fibre, from damage. [Increasingly, acrylates, e.g. urethane acrylane, are being used in place of nylon.] Again, the physical properties of the material are very important in terms of their effect on the optical performance of the fibre, particularly its temperature sensitivity. Particularly now that optical fibres are being more widely deployed, it is important that optical fibres can be packaged to withstand extremes of temperature. In practice, it is insensitivity to low temperatures, e.g. sub-zero centigrade, which is the most difficult to achieve. For network use in continental climates, it is desirable that optical fibres should show no significant excess loss at temperatures as low as -20.degree., -40.degree. or even -60.degree. C. Some relevant aspects of the temperature sensitivity of optical fibres are dealt with in the following papers:
T. A. Lenahen, A. T. & T Tech. J. , V. 64, No. 7, 1985, pp 1565-1584,
T. Yabuta, N. Yoshizawa and K. Ishihara, Applied Optics, V.
REFERENCES:
patent: 5042907 (1991-08-01), Bell et al.
patent: 5109456 (1992-04-01), Sano et al.
Suzuki et al, "Fine and Tough Optical Cords with Polyetherimide Coated Fibers", 5th International Conference on Plastics in Telecommunications, 1989, London, pp. 12/1-12/7 (No Month).
Barker Philip A.
Fisk Christopher
Jenkins Peter D.
Stockton David J.
British Telecommunications public limited company
Lee John D.
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