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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.
Two beams from a laser are injected into the same fibre 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.The development of
diode 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.
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 ShorthillV.Vali, R.W. Shorthill, ‘Fiber Ring Interferometer, Appl Optics’, 15...
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