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Fibre optic gyroscope
A fibre optic gyroscope (FOG) senses changes in orientation, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre which can be as long as 5 km.
The development of diode (semiconductor) lasers and low-loss single-mode optical fibre in the early 1970s for the telecommunications industry enabled Sagnac effect fibre optic gyros to be developed as practical devices.
==Operation==
Two beams from a laser are injected into the same fiber but in opposite directions. Due to the Sagnac effect, the beam travelling against the rotation experiences a slightly shorter path delay than the other beam. The resulting differential phase shift is measured through interferometry, thus translating one component of the angular velocity into a shift of the interference pattern which is measured photometrically.
Beam splitting optics launches light from a laser diode into two waves propagating in the clockwise and anticlockwise directions through a coil consisting of many turns of optical fibre. 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 number of turns in the coil. The FOG was first proposed by Vali and Shorthill〔V.Vali, R.W. Shorthill, ‘Fiber Ring Interferometer, Appl Optics’, 15 (1976)〕 in 1976. Development of both the passive interferometer type of FOG, or IFOG, and a newer concept, the passive ring resonator FOG, or RFOG, is proceeding in many companies and establishments worldwide.〔Hervé Lefèvre, "The Fiber-Optic Gyroscope", 1993, ARTECH HOUSE, INC. ISBN 0-89006-537-3〕
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