Fibre Optic Gyros (FOGs) consist of a coil of fibre optic cable in which two light beams travel through the entire cable length in opposite directions and are then combined. The development of low loss, single mode, optical fibre in the early 1970’s, enabled Sagnac effect fibre optic gyros to be developed. The sensor operates on a similar principle to the RLG, where the interference pattern created from the counter-propagating light waves, after travelling through the fibre, is a measure of the angular rotation of the device. However, they differ in that an incoherent broadband light source is used.
Fibre optic gyros tend to be packaged in cylindrical containers, for example 10cm diameter by 2.0cm deep. A sensor may contain as much as 5 kilometres of fibre. FOG sensitivity is a function of coil radius (enclosed area) and optical path length, so once again larger sensors tend to be more accurate sensors.
FOG performance in general, is similar to but not quite as good as RLG. Bias stabilities of 0.1 deg/hr or better and random walks of 0.005 deg/rt(hr) would be typical. RLG has inherently better scale factor stability.
Outside of the oilfield, FOG sensors tend to be used for similar applications to RLGs, but for those applications where environmental conditions and accuracy is less important and where cost is a factor. Although experimental devises have been developed, no FOG system has been commercially marketed within the oilfield. Once again sensor size and temperature concerns are limiting factors. In general FOG performance is more sensitive than RLGs to environmental conditions such as shock, vibration and temperature gradients.
Large diameter, high accuracy systems have been developed and are in use in space and submarine applications, where size restrictions are secondary.
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