The principle of the FOG is based on the Sagnac effect, that is, the same two light beams emitted by the same light source propagate in the opposite direction and converge at the same observation point. If there is an angular velocity with respect to the inertial space perpendicular to the axis of the plane where the closed optical path is located, the travel distance of the two rays in the forward and reverse directions is different. An optical path difference occurs. The optical path difference is proportional to the angular velocity of the rotation, so we know the information between the optical path difference and phase difference, and we can get the rotational angular velocity.
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