Varnham, Malcolm Paul (1984) A study of highly-birefringent optical fibres. University of Southampton, Doctoral Thesis.
Abstract
A study has been undertaken of the birefringence induced in optical fibres by thermal stress. A new theory predicts an optimal structure, called the Bow-Tie fibre, which maximises the birefringence for a given mismatch in thermal expansion coefficient. An extension of the analysis relates the dependence of birefringence to the V-value and explains why the retardation in birefringent fibres changes upon stretching. The applicability of the standard refractive-index profiling technique to Bow-Tie preforms has been studied. It is shown experimentally that focussing problems aree small; and an analysis of the effect of thermal stress on the reconstructed profile is presented. The performance of the Bow-Tie fibre is compared with other high birefringence designs. Higher values of birefringence and better polarisation maintenance has been demonstrated with the Bow-Tie fibre. Techniques for realising a truly single-polarisation fibre are reviewed and a multimode fibre, called a stress-guide, has been designed and fabricated. Bow-Tie fibres have been operated in a truly single-polarisation-mode regime and their properties measured. Higher-order modes in Bow-Tie fibres display non-degeneracy in both orientation and polarisation. Snyder and Ruhl have predicted that a new form of mode leakage occurs in high-birefringence fibres when operated at low V-values. Their theoretical formulation is investigated and their leakage mechanism is shown to be an artifact of an inappropriate model. An alternative analysis of the polarising properties of Bow-Tie fibres is presented using perturbation theory. There is good agreement with experiment. It is shown that the ability of high-birefringence fibres to transmit polarised light is limited by the field curvature of the fundamental mode and by mode-coupling.
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