Optical fibres for communications : manufacture, properties, and application
The successful application of optical fibres to wideband communications demands the utilisation of fibres exhibiting very low attenuation and high bandwidth at the wavelength of operation. This thesis describes an experimental study of the homogeneous chemical vapour deposition (HCVD) technique, a convenient process for the manufacture of ultra-pure waveguide glasses. Objectives of the programme included the development of technologies which could be adopted by industry for the routine manufacture of high-quality optical fibres. Aspects of the work have included not only optimisation of the HCVD and fibre-driving processes but also the development of protective coatings which preserve the physical and optical performance of the fibres. A lathe-based system has been constructed for the manufacture of optical-fibre preforms by HCVD, and evaluation of fibres prepared by HCVD has led to the development of ternary phosphogermanosilicate glasses for use as the core glass in multimode fibres. The fibre drawing operation is described, and, in particular, it is shown that the addition of a fibre diameter measuring system led to important refinements to the preform-manufacturing and fibre-drawing processes. Multimode graded-index fibres have been produced in lengths in excess of 5km, with numerical apertures up to 0.25, attenuation as low as 0.8 dB/km, and intermodal dispersion as low as 0.3 ns/km; fibres suitable for operation in the 1.3µm and 1.55µm regions have been prepared. The versatility of the HCVD process has enabled low-loss monomode fibres to be fabricated. In addition, a monomode fibre having a linear birefringence of less than 3°/m has been manufactured for transducer applications; this represents the lowest value reported to date for the birefringence of an optical fibre. Techniques have been developed for the application of a surface-protective coating to fibres in-line with drawing. Using a thick primary coating of a thermosetting silicone elastomer, it has been possible to maintain the high inherent strength of freshly-drawn fibres without degrading their optical performance; a median tensile strength of 5.2 GN/m2 was obtained in 100m gauge-length samples. An experimental evaluation of materials suitable for providing a tough outer jacket over primary-coated fibres is described. An extrusion line has been developed for applying thermoplastic jackets, and by using nylon-6 as the jacketing material, it has been possible to provide an excellent degree of microbending resistance and mechanical protection without compromising the fibres' performance. Suitably protected graded-index fibres have been cabled and installed in a particularly severe environment, where they form part of an optical link transmitting video signals and high bit-rate digital data. The performance of the fibres within the link surpassed initial expectations, and confirmed the capabilities of the fibre manufacturing technology.