How does a ring laser gyroscope work?

Ring laser gyroscopes are the result of over 100 years of research, development and experimenting in the field of navigation technology. They are essential to flight safety, reduced human error, accuracy and more, in both manned and unmanned aerial vehicles.

While the first laser gyro was experimentally demonstrated in 1963, by W.M. Macek and D.T.M. Davis, the very first usable gyrocompass goes back as far as 1904 and was invented by German inventor Hermann Anschütz-Kaempfe. Gyroscopes have come a long way since then, evolving from mechanical to optical self-contained laser technology such as the Honeywell Ring Laser Gyroscope or RLG.

What is a ring laser gyroscope?

Gyroscopes are devices utilized for measuring and maintaining orientation of an object in inertial space, at any given time. Before RLG technology, mechanical gyroscopes were the norm. A mechanical gyroscope is based on the principle of conservation of angular momentum, which states that if no external torque acts on a system, the total angular momentum of the system remains constant. What this means is that a rotating object will keep spinning on an axis if no external torque is applied. Angular momentum is an essential physics characteristic which cannot be created or destroyed, only transferred.

Mechanical gyroscopes consist of a disc, or spinning wheel, with an axle that assumes any orientation. Orientation changes rapidly when external torque is applied, however when the gyro is mounted in a gimbal, torque is minimized and the spin axis defined by the axle is thus stabilized.   

Ring laser gyroscopes are today’s industry standard and abide by the Sagnac effect to sense orientation, which manifests itself in a ring interferometer. The interferometer is where a ring laser gyro is initially set up. Interferometers are investigative tools that operate by superimposing two or more light sources to create an interference pattern, which is then tracked and analyzed. Interferometers are typically used for highly sensitive measurements which cannot be achieved in other ways, such as identifying variations on microscopic organisms or detecting gravitational waves, and have wide applicability. 

The interferometer used for the ring laser gyro comprises narrow tunnels that form a closed circle surrounding a block of zero expansion glass, which is made of lithium oxides, aluminum and silicon. To create the interference pattern for the Sagnac effect, three mirrors are placed at each vertex and two counter-propagating laser beams are formed in the active cavity. 

Although the beams run in different, opposite directions, they enter and exit at the exact same point, which enables the interferometer to measure the reassembled signal at the moment of the exit.

When a ring laser gyro is in motion, the beams of light travel different distances.  The difference in frequency is proportional to the rotation rate. The frequency difference is measured via an interference fringe pattern whose phasing contains the directional information.