Linkage between polarisation characteristics over two different patterns of instantaneous large DGD occurrence

Through long-term field fibre PMD measurement, two occurrence patterns of large instantaneous differential group delays (DGDs), of less than 10^-5 probability, were observed. Most high DGD occurrences, which continue more than hours in limited bandwidth, clearly show positive correlation with ambient temperature even though most of the fibres are buried; and only an emergent spike, which seems not to relate to temperature, was also recorded. In both patterns, second-order polarisation mode dispersion on the wavelength at the same timing has local maximum values with the same occurrence patterns, but PDL does not have local maximum values on the wavelength at the same timing.