A fiber optic system comprising: a fiber optic gyroscope comprising: a light source, a fiber coil, an optical device comprising a phase modulator and being configured to split a light beam coming from the light source by means of beam splitting and to modulate the phase of at least one of the generated light beams to adjust a current working point of the fiber optic gyroscope, to irradiate the generated light beams in directions opposite to each other into the fiber coil, and to unify the light beams leaving the fiber coil, a photo detector that is configured to generate a signal corresponding to the light intensity of an interference signal generated due to the reunification of the light beams, an amplifier that is configured to amplify the signal generated in the photo detector, and an evaluation unit that is configured to generate by means of the signals generated by the amplifier a modulation signal, which controls the phase modulator such that a reset of the interference signal according to the current working point of the fiber optic gyroscope is obtained or that a predetermined value is set as the current working point of the fiber optic gyroscope, and to determine based on the modulation signal a rotation rate of the fiber optic gyroscope, and a control device that is configured to control at least one element of the fiber optic gyroscope such that at least two different values (Ω 1, Ω 2) for the rotation rate are determined by the evaluation unit for almost constant rotation rate applied to the fiber optic gyroscope; wherein the system comprises further a processing unit that is configured to determine the size of the bias error of the fiber optic gyroscope by comparison of the at least two determined values (Ω 1, Ω 2) for the rotation rate. a fiber optic gyroscope comprising: a light source, a fiber coil, an optical device comprising a phase modulator and being configured to split a light beam coming from the light source by means of beam splitting and to modulate the phase of at least one of the generated light beams to adjust a current working point of the fiber optic gyroscope, to irradiate the generated light beams in directions opposite to each other into the fiber coil, and to unify the light beams leaving the fiber coil, a photo detector that is configured to generate a signal corresponding to the light intensity of an interference signal generated due to the reunification of the light beams, an amplifier that is configured to amplify the signal generated in the photo detector, and an evaluation unit that is configured to generate by means of the signals generated by the amplifier a modulation signal, which controls the phase modulator such that a reset of the interference signal according to the current working point of the fiber optic gyroscope is obtained or that a predetermined value is set as the current working point of the fiber optic gyroscope, and to determine based on the modulation signal a rotation rate of the fiber optic gyroscope, and a light source, a fiber coil, an optical device comprising a phase modulator and being configured to split a light beam coming from the light source by means of beam splitting and to modulate the phase of at least one of the generated light beams to adjust a current working point of the fiber optic gyroscope, to irradiate the generated light beams in directions opposite to each other into the fiber coil, and to unify the light beams leaving the fiber coil, a photo detector that is configured to generate a signal corresponding to the light intensity of an interference signal generated due to the reunification of the light beams, an amplifier that is configured to amplify the signal generated in the photo detector, and an evaluation unit that is configured to generate by means of the signals generated by the amplifier a modulation signal, which controls the phase modulator such that a reset of the interference signal according to the current working point of the fiber optic gyroscope is obtained or that a predetermined value is set as the current working point of the fiber optic gyroscope, and to determine based on the modulation signal a rotation rate of the fiber optic gyroscope, and a control device that is configured to control at least one element of the fiber optic gyroscope such that at least two different values (Ω 1, Ω 2) for the rotation rate are determined by the evaluation unit for almost constant rotation rate applied to the fiber optic gyroscope; wherein the system comprises further a processing unit that is configured to determine the size of the bias error of the fiber optic gyroscope by comparison of the at least two determined values (Ω 1, Ω 2) for the rotation rate.