 Ring laser gyroscopes ( <unk> ) and fibre optic gyroscopes ( <unk> ) are interferometers used in navigation systems . They operate on the principle of the Sagnac effect . The distinction between <unk> and <unk> is that in a <unk> , the entire ring is part of the laser while in a <unk> , 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 <unk> , the observed phase shift is proportional to the accumulated rotation , while in a <unk> , the observed phase shift is proportional to the angular velocity . 
