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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.
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 a 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 resonant frequency with respect to the light travelling in the other direction, resulting in the interference pattern beating at the detector. The angular position is measured by counting the interference fringes.
RLGs, while more accurate than mechanical gyros suffer from an effect known as "lock-in". When the ring laser is rotating very slowly, the frequencies of the counter-rotating lasers become very close to each other. 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 the detector, which causes the device to no longer track angular position.
This effect is compensated by adding dithering. 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.
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 Inertial Reference System. In these applications, it has replaced its mechanical counterpart, the Inertial guidance system.
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