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A ring laser gyroscope (RLG) uses interference of laser light within an optical ring to detect changes in orientation and spin. It is an example of a Sagnac interferometer.

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Description

The first experimental ring laser gyroscope was demonstrated in the US by Macek and Davis in 1963.[1] The technology has since been developed by a number of companies and establishments world-wide. Many tens of thousands of RLGs are operating in inertial navigation systems and have established high accuracy, with better than 0.01°/hour bias uncertainty, and mean time between failures in excess of 60,000 hours.

Schematic representation of a ring laser setup. At the beam sampling location, a fraction of each of the counterpropagating beams exits the laser cavity.

Ring laser gyroscopes 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 gyroscope, 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.

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 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 rotated clockwise and anti-clockwise 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. 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 device is not rotating and lock-in can occur. In a technically more complicated solution the gyro assembly is not rotated back and forth, but in one direction only at a constant angular rate.

A related device is the fibre optic gyroscope which operates similarly to the ring gyro, but typically has a longer optical circuit with fewer mirrors or prisms, because the transmission paths are within a coiled optical fiber. A fiber gyroscope doesn't necessarily use fiber laser technology; the laser action may be located outside a passive ring on a beam splitter port.

Primary applications include navigation systems on commercial airliners, ships and spacecraft, where RLGs are often referred to as Air Data Inertial Reference Units. In these applications, it has replaced its mechanical counterpart, the Inertial guidance system.

Examples of RLG applications

See also

References

External links








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