Polarization mode coupling in long single-mode fibres

In this paper we look at the polarization state evolution of light propagating in long, birefringent, single-mode fibres in the presence of mode coupling. We show first that there always exist two orthogonal linear input polarizations which lead to linear output polarization for monochromatic light. The applicability of the coupled power equations is discussed and it is shown that they cannot be used with only two propagating modes. A more detailed analysis is then carried out for a uniform coupling and the effect of the spectral width of the source is investigated. Some insight into the time-dependent propagation is also given. Experimental results illustrate the theoretical considerations.