A Sagnac interferometer consists of an optical ring cavity in which two light beams (usually laser beams) are propagating in opposite directions. They interfere at a beam splitter and as the length of the optical path of the ring cavity is the same no matter whether the light goes round clockwise or anti-clockwise, and hence the two beams will always interfere with the same phase.

If, however, the interferometer is rotating, then the light which goes round in the direction of rotation will have a shorter distance (as the mirrors of the cavity are moving towards it) than the other light beam (which experiences the mirrors as receding), and the phase will be different.

Hence, if one monitors the intensity at the output ports of the beam splitter, one can detect changes in the angular velocity of the interferometer.

Instead of using mirrors to form a closed ring cavity, one can also use an optical fibre closed to a ring by a fibre coupler. In this way the device is usually made for its most important application, the laser gyroscope.