File:Zero-Area Sagnac Interferometer.svg

English: The fringe shift in a Sagnac interferometer due to rotation has a magnitude proportional to the enclosed area of the light path, and this area must be specified in relation to the axis of rotation. Thus the sign of the area of a loop is reversed when the loop is wound in the opposite direction (clockwise or anti-clockwise). A light path that includes loops in both directions, therefore, has a net area given by the difference between the areas of the clockwise and anti-clockwise loops. The special case of two equal but opposite loops is called a Zero-area Sagnac interferometer. The result is an interferometer that exhibits the exceptional stability of the Sagnac topology while being insensitive to rotation.