The Sagnac effect

Laser gyroscope registers frequency differences

In a ring laser, several mirrors form a closed circuit or ring – also known as a resonator – around which the beams travel. This circuit, which is typically rectangular, is encased in a steel tube in which a helium and neon gas mixture is stimulated with high frequency radio waves to emit laser light of a certain wavelength. This laser light can now travel round the circuit in two opposite directions – one clockwise and the other anti-clockwise. When the array is stationary, the two laser beams traveling in opposite directions meet exactly at the point of origin in each case. When the array is rotating, however, because it is rotating as part of the rotation of the earth for example, one beam has a shorter distance to travel while the other has some distance to “catch up” to reach the point of origin. As a consequence of this, a minimal frequency difference occurs between the two directions of travel. This is called the Sagnac effect. The difference in travel time as a measurement of the rotation speed of the reference system is proportional to that of the surface enclosed in the ring laser – that is why the size of the ring laser is a crucial factor. With 16 square meters of surface traveled, the Wettzell ring laser is the largest and most accurate in the world.