A ring laser gyroscope uses interference of laser light within a bulk optic ring to detect changes in orientation and spin. It is an application of a Sagnac interferometer.
The first experimental ring laser gyro was demonstrated in the US by Macek and Davis in 1963 and has since been developed by a number of companies and establishments world-wide to large scale production status. Many tens of thousands of RLG's are operating in strap-down INS systems and have established high accuracy with better than 0.01o/hour bias uncertainity and MTBFs in excess of 60,000 hours. The RLG is basically an active resonant system with the laser cavity forming the closed optical path. Input rotation rates are measured by the difference in the resonant frequencies of the clockwise and anticlockwise paths resulting from the difference in the path lengths produced by the rotation.
Ring laser gyros (RLG) can be used as the stable elements (for one degree of freedom each) in an inertial reference system. The advantage of using an RLG is that there are no moving parts. Compared to the conventional spinning gyro, this means there is no friction, which in turn means there will be no inherent drift terms. Additionally, the entire unit is compact, lightweight and virtually indestructible, meaning it can be used in aircraft. Unlike a mechanical gyroscope, the device does not resist changes to its orientation. Physically, an RLG is composed of segments of transmission paths configured as either a square or a triangle and connected with mirrors. One of the mirrors will be partially silvered, allowing light through to the detectors. A laser is launched into the transmission path in both directions, establishing a standing wave resonant with the length of the path. As the apparatus rotates, light in one branch travels a different distance than the other branch, changing its phase and resonance frequency with respect to the light travelling in the other direction, resulting in the interference pattern beating at the detector. The angular rate is measured by counting the interference fringes.
RLGs, while more accurate than mechanical gyros, suffer from an effect known as "lock-in" at very slow rotation rates. When the ring laser is rotating very slowly, the frequencies of the counter-rotating lasers become very close (within the laser bandwidth). At this low rotation, the nulls in the standing wave tend to "get stuck" on the mirrors, locking the frequency of each beam to the same value, and the interference fringes no longer move relative to the detector; in this scenario, the device will not accurately track its angular position over time.
Dithering can compensate for lock-in. The entire apparatus is twisted and untwisted about its axis at a rate convenient to the mechanical resonance of the system, thus ensuring 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.
A related device is the fiber optic gyroscope which operates similarly to the ring gyro, but implementing transmission paths with a coiled fiber optic cable.
Primary applications include navigation systems on commercial airliners, ships and spacecraft, where RLGs are often referred to as an Inertial Reference System. In these applications, it has replaced its mechanical counterpart, the Inertial guidance system.
