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The Sagnac effect (also called Sagnac interference), named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer. A beam of light is split and the two beams are made to follow the same path but in opposite directions. To act as a ring the trajectory must enclose an area. On return to the point of entry the two light beams are allowed to exit the ring and undergo interference. The relative phases of the two exiting beams, and thus the position of the interference fringes, are shifted according to the angular velocity of the apparatus. This arrangement is also called a Sagnac interferometer. A gimbal mounted mechanical gyroscope remains pointing in the same direction after spinning up, and thus can be used as a rotational reference for an inertial navigation system. With the development of so-called laser gyroscopes and fiber optic gyroscopes based on the Sagnac effect, the bulky mechanical gyroscope is replaced by one having no moving parts in many modern inertial navigation systems. The principles behind the two devices are different, however. A conventional gyroscope relies on the principle of conservation of angular momentum whereas the sensitivity of the ring interferometer to rotation arises from the invariance of the speed of light for all inertial frames of reference. ==Description and operation== Typically 3 or more mirrors are used, so that counter-propagating light beams follow a closed path such as a triangle or square.(Fig. 1) Alternatively fiber optics can be employed to guide the light through a closed path.(Fig. 2) If the platform on which the ring interferometer is mounted is rotating, the interference fringes are displaced compared to their position when the platform is not rotating. The amount of displacement is proportional to the angular velocity of the rotating platform. The axis of rotation does not have to be inside the enclosed area. The Sagnac effect in a circular loop can be understood on an intuitive level as follows. When the loop is rotating, the point of entry/exit moves during the transit time of the light. The backwards-propagating beam covers less distance than the forwards-propagating beam and arrives earlier.(Fig. 3) This creates a shift in the interference pattern. The shift of the interference fringes is thereby proportional to the platform's angular velocity. This simplistic explanation, however, breaks down in cases where the light is propagating through a medium which has a refractive index that is not one. In that case, relativistic addition of velocities can be used to calculate the lab frame phase velocity of the light moving in the same direction as the rotation as well as for the light moving in the opposite direction from the rotation. The difference in lab frame phase velocities determines the difference in travel times, and this difference in travel times can be multiplied by the optical frequency to determine a phase difference. The rotation thus measured is an absolute rotation, that is, the platform's rotation with respect to an inertial reference frame. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sagnac effect」の詳細全文を読む スポンサード リンク
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