Ring Laser Gyroscope - Principle of Operation

Principle of Operation

A certain rate of rotation induces a small difference between the time it takes light to traverse the ring in the two directions according to the Sagnac effect. This introduces a tiny separation between the frequencies of the counter-propagating beams, a motion of the standing wave pattern within the ring, and thus a beat pattern when those two beams are interfered outside the ring. Therefore the net shift of that interference pattern follows the rotation of the unit in the plane of the ring.

RLGs, while more accurate than mechanical gyroscopes, suffer from an effect known as "lock-in" at very slow rotation rates. When the ring laser is hardly rotating, the frequencies of the counter-propagating laser modes become almost identical. In this case, crosstalk between the counter-propagating beams can allow for injection locking so that the standing wave "gets stuck" in a preferred phase, thus locking the frequency of each beam to each other rather than responding to gradual rotation.

Forced dithering can largely overcome this problem. The ring laser cavity is rotated clockwise and anti-clockwise about its axis using a mechanical spring driven at its resonance frequency. This ensures that the angular velocity of the system is usually far from the lock-in threshold. Typical rates are 400 Hz, with a peak dither velocity of 1 arc-second per second. Dither does not fix the lock-in problem completely, as each time the direction of rotation is reversed, a short time interval exists in which the rotation rate is near zero and lock-in can briefly occur. If a pure frequency oscillation is maintained, these small lock-in intervals can accumulate. This was remedied by introducing noise to the 400 Hz vibration.

A related device is the fibre optic gyroscope which also operates on the basis of the Sagnac effect, but in which the ring is not a part of the laser. Rather, 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 when interfered after their pass through the fiber ring proportional to the rate of rotation. This is therefore less sensitive than the RLG in which the externally observed phase shift is proportional to the accumulated rotation itself, not its derivative. However the sensitivity of the fiber gyro is enhanced by having a long optical fiber coiled for compactness, but in which the Sagnac effect is multiplied according to the number of turns.