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Transient polarisation effects as the cause of the drift of a signal from a ring fibre interferometer

It is shown theoretically and experimentally that one reason for the appearance of a parasitic signal at the output of a fibre Sagnac interferometer, assembled in the conventional manner, may be the effects associated with the anisotropy of a phase modulator, which lead to modulation of the polarisation. These effects are transient. They depend on the delay of phase modulation of eigenwaves travelling in one direction, relative to the modulation of counterpropagating waves, because of an asymmetric position of the modulator in the fibre ring, which is typical of conventional configurations. The results obtained make it possible to explain the experimentally observed effects and to recommend ways of avoiding them.