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A ring laser gyroscope is a precision instrument that uses a laser beam traveling in two directions to measure changes in angle, or a direction. Gyroscopes are used in navigation systems for aircraft and ships, and for guidance systems in missiles and precision weapons. The principle of using light to measure changes in direction are based on research by French scientist Georges Sagnac performed in 1913.
Gyroscopes use the principle of inertia to determine direction or changes in position. A spinning gyroscope wheel wants to remain in one position and will resist being turned. This can be demonstrated by a spinning top that will resist being pushed to one side, or attempting to turn a spinning bicycle wheel to one side.
A ring laser gyroscope makes use of the Doppler principle to measure differences in laser light beams. In 1842, Christian Doppler found that the frequency of sound appears different to a listener if the source of the sound is moving. Sounds moving toward a listener appear higher, and moving away appear lower in frequency. The effect also occurs with light, and a laser gyroscope utilizes this principle because the two beams travel at slightly different distances when the gyroscope is moved or tilted, as found by Sagnac.
The design of a ring laser gyroscope is normally a triangle with three equal sides, or an equal-sided box. A helium laser is placed on one side of the triangle or box, and laser beams are sent in opposite directions around the triangle. Using mirrors and prisms, the two beams are sent to a detector that looks at both the light and dark lines formed by the two beams, called interference patterns. The detector can look for changes in the interference patterns, which will move or shift position if the gyroscope is moved.
When the gyroscope is level, the two laser beams return to the detector at a known time difference, and the interference patterns are stationary. Tilting the ring laser gyroscope to one side causes the laser beams to return at slightly different times, and the interference patterns move at a rate consistent with the amount of tilt. The detector can be calibrated to show a tilt measurement for a turn-and-bank indicator on an aircraft used for precision turns, or to turn a compass dial used for navigation called a directional gyro.
Ring laser gyroscope technology began replacing mechanical gyroscopes in the late 20th century. Prior to that time, gyroscopes used wheels spun at very high speeds to create a stable gyroscope effect. These gyroscopes required compressed air or electricity for power, and were subject to performance losses due to mechanical friction. The ring laser gyroscope has no moving parts, and once calibrated can give excellent accuracy with minimal performance loss.
A problem with early laser gyroscopes was difficulty in measuring very small changes in direction or tilt. This effect is called lock-in, and the two laser beams appear at the detector at the same time increment as a non-moving gyroscope, which is incorrectly interpreted as being level. One method to prevent this error, called mechanical dithering, uses a vibrating spring to move the detector at a specific rate to prevent lock-in. Another method spins the gyroscope at a specific rate to prevent the false level measurements, though this unit is more expensive to produce.