Ragdale, Catherine Mary (1982) Properties of single-mode optical fibres. University of Southampton, Doctoral Thesis.
Abstract
Several aspects of propagation in single-mode fibres are discussed. The radiation, loss of the fundamental mode arising from bends in the fibre is studied. It is shown that for an accurate prediction of bending loss both the transition loss and pure-bend loss should be considered. A coupled mode theory is proposed which accounts well for the transition loss, but less exact, simple formulae are also derived for the mode-shift and transition loss at a bend. The losses of the fundamental LP p~ mode and second-order LP11 mode arising from helical bending and random bending are calculated. The loss of the LP11 mode is shown to be very high near cut-off, thus quasi-single mode operation is possible for wavelengths below the theoretical limit. The dispersive properties of single-mode fibres are studied. In the single-mode regime a significant amount of power is carried in the cladding, thus it is shown that the material dispersions of both the core and cladding are important. In addition the material, waveguide and profile dispersive effects are closely interrelated. It is found that the total first-order chromatic dispersion falls to zero in a wavelength range > 1.3µm the exact value being selected by changing either the core diameter or the fibre NA. Some practical aspects of designing single-mode fibres to operate with zero first-order dispersion are discussed, including the dependence of the bend and joint losses on the fibre parameters necessary for zero dispersion and the control of the core diameter needed to maintain negligible dispersion. The limitation imposed on the bandwidth by ellipticity and stress-induced birefringence is considered. The inconsistencies between previous theories for predicting ellipticity birefringence are discussed and a more accurate analysis is presented.
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