 Ring laser gyroscopes ( RLGs ) and fibre optic gyroscopes ( FOGs ) are interferometers used in navigation systems . They operate on the principle of the Sagnac effect . The distinction between RLGs and FOGs is that in a RLG , the entire ring is part of the laser while in a FOG , an external laser injects counter @-@ propagating beams into an optical fiber ring , and rotation of the system then causes a relative phase shift between those beams . In a RLG , the observed phase shift is proportional to the accumulated rotation , while in a FOG , the observed phase shift is proportional to the angular velocity . 
