Ring laser gyroscope
A ring laser gyroscope consists of a ring laser
Ring laser
A ring laser is a laser in which the laser cavity has the shape of a ring. Light in ring lasers has two possible directions of propagation: clockwise and counter-clockwise....

 having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect
Sagnac effect
The Sagnac effect , 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 ring interferometry. A beam of light is split and the two beams are made to follow a trajectory in...

 which shifts the nulls of the internal standing wave pattern in response to angular rotation. Interference between the counter-propagating beams, observed externally, reflects shifts in that standing wave pattern, and thus rotation.
The first experimental ring laser gyroscope was demonstrated in the US by Macek and Davis in 1963.