Refer to figure.
The ring laser gyro (RLG) is a modern, more sophisticated alternative to the conventional gyroscope. It is used for the aircraft inertial navigation systems and attitude/heading reference systems. It is a very accurate system to measure rotation about a given axis and is widespread in modern commercial aircraft IRS systems.

Conventional gyro’s rotation mass is replaced by two contra-rotating beams of light. The case of the RLG is made of a vitreous ceramic material of low-temperature coefficient. Two gas-tight openings are carefully designed into the single-piece case of the RLG, which are then filled with inert gas, such as a mixture of Helium and Neon. A direct current electrical discharge ionizes the gas and creates two beams of light flowing in opposite directions in the two cavities of the RLG case. In order to produce a continuous laser circuit in the enclosed area of the RLG’s case, a set of mirrors is installed to reflect and trap the laser beams. The frequency of oscillation of each beam depends on the optical path length and corresponds to the cavity resonance condition.

When the RLG is stationary, both optical path lengths of each beam are identical. However, when the RLG is rotated about the axis perpendicular to the laser beams plane, one beam will travel a larger distance, and the other will cover a shorter distance. The frequency difference induced by different path lengths is proportional to the rotation rate.