Polarization noise in single mode fibres and its reduction by depolarizers

Fluctuations of the state of polarization in single mode fibres due to environmental conditions such as vibrations cause intensity noise if elements with polarization-dependent loss are inserted into the fibre system. Diffraction gratings, which are key elements for wavelength division multiplex-single mode fibre systems, exhibit a strong polarization dependence of loss. The polarization noise, originating from the combination of single mode fibres and diffraction gratings, is investigated experimentally. A drastic deterioration of the signal/noise ratio is observed, which is even more severe than modal noise with multimode fibres. Depolarizing devices, which reduce this kind of noise sufficiently, are presented and examined.