Nonlinear effects in optical fibres

Abstract

This thesis presents a study of the nonlinear effects in optical fibres brought about by the real part of the third-order susceptibility. In single-mode fibres nonlinear interactions between parallel polarized waves are shown to develop an amplitude-phase interplay when generation and amplification of optical frequencies occur. For a given fibre length the optimization of these processes requires a certain input relative phase and a finite degree of phase mismatch. A generalization of the amplitude-phase interplay to a continuum of spectral lines reveals the mechanism of spectral broadening. It is shown how the nonlinear variation of the phase spectrum broadens the pulse, while the spread of the amplitude distribution results in a pulse width reduction. In low birefringence fibres the nonlinear coupling of power between the polarization eigenmodes is shown to contribute to intensity discrimination of optical waves. In high birefringence fibres the strong phase-mismatch can be circumvented by means of counter-propagating waves of the same frequency or two copropagating waves of different frequencies. In the case of quasi-monochromatic waves propagating down a birefringent fibre the nonlinearly induced variations of the spectral amplitude and phase distributions result in deterioration of the degree of coherence. This quantity will depend on the level of power and its partition between the polarization eigenmodes. Show more