Abstract: A small modulation current is applied to the light source of a Sagnac-type fiber optic gyroscope to increase accuracy and/or stability of scale factor (open loop configuration) and to reduce bias errors and random walk (closed-loop configuration). The phase and amplitude of the modulation current are controlled through a control circuit whose reference is picked off at a first beamsplitter in series with the light source. In such a way, light emitted from the light source into the interferometer contains no modulation. The elimination of modulation removes a source that would otherwise cause deterioration of gyroscope stability resulting from light returning from the interferometer or due to electrical interference, thus limiting measurement accuracy.