Measurement of angular velocity is useful in many different applications, from missile navigation to motion control. There are three broad categories of possible sensors for angular velocity: Ring Laser Gyroscopes (RLG), Fiber Optic Gyroscopes (FOG), and MEMs-based Gyroscopes. The former two utilize the Sagnac effect to measure velocity and are much more sensitive compared to MEMs gyroscopes. A fiber optic gyroscope (FOG) is a device that senses changes in orientation using the Sagnac effect. Such a fiber optic device performs the function of a mechanical gyroscope. However, it should be noted, its fiber optic solution is instead based on the interference of light which has passed through a coil of optical fiber that can be as long as 5 km.
What is the principle of the fiber optic gyroscope? Two laser beams are injected into the same optical fiber but in opposite directions. The beam that moves against the rotation experiences a slightly shorter path delay than the other beam due to the Sagnac effect. The resulting differential phase shift is measured through interferometry. This phase shift translates one component of the angular velocity into a shift of the interference pattern which is measured photometrically. The strength of the Sagnac effect is dependent on the effective area of the closed optical path: this is not simply the geometric area of the loop but is enhanced by the member of turns in the coil.
Victor Vali and Richard Shorthill demonstrated an operational fiber optic gyroscope for the first time in 1976. In that same year, McDonnell Douglas Astronautics Co. in Huntington Beach, CA, completed a project to redesign a new, lower-cost inertial measurement unit for the Delta rocket based on dry-tuned mechanical gyros. The technology supporting early efforts in the field of fiber optic gyroscopes were fiber optic products derived from the Sagnac interferometer. The first sensor of this type was the Sagnac acoustic sensor. Another derivative sensor is the Sagnac strain sensor. Nowadays, the Sagnac interferometer continues to be a useful tool for a variety of sensing and communication applications. One principal advantage of the Sagnac acoustic and distributed sensors is that they can be supported by very low-cost single-mode optical fiber. This opens up a number of interesting applications, including identifying leaks in pressurized pipes and containers, identifying the location of insects in grain storage facilities, and locating termites in wood.
Fiber optic gyroscopes are robust apparatus for measuring angular velocity. It is interesting that fiber optic gyroscopes are instruments where merely improving the processing of the incoming sensor signal can yield more stability, linearity, and sensitivity.
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