This Demonstration simulates the operation of a Sagnac interferometer with three mirrors and two input beams. The interference between the output beams depends on the placement of the components. This differs from the two-mirror Sagnac interferometer in that the output beams are no longer in parallel when the mirrors are tilted.
For comparison, see the Demonstration "Sagnac Interferometer". This three-mirror Sagnac-loop is widely used in measurement of weak signals, as a super MZ interferometer, and so on, in experimental quantum optics.