Abstract: The object of the present invention is a measurement method and a system for measurement of an amplitudes ratio of two first harmonics of a signal obtained from the Sagnac fibre-optic inter-ferometer system as well as the Sagnac fibre-optic system, and an application of the said system to measure the amplitude ratio of two first harmonics obtained from the Sagnac fibre-optic system and the Sagnac fibre-optic interferometer system for detection of rotational movements, in particular seismic rotational movements and rotational movements of structures.