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Incorporating the Sagnac effect, optically-based gyros offer economical, robust, reliable performance.
Traditional gyroscopes, based on the momentum of a spinning rotor, are used in a wide range of applications in which orientation and guidance data are required. Although they are time-tested and well understood, these mechanical devices are susceptible to damage from shock and vibration and exhibit cross-axis acceleration sensitivity. Lower cost versions may also suffer from reliability problems. In an attempt to solve these problems, engineers have developed alternative technologies such as ring laser gyros (RLGs), fiberoptic gyros (FOGs), and Coriolis vibratory gyros (CVGs).
Both the RLG and FOG employ the Sagnac effect, discovered in 1913. In a Sagnac interferometer, two light beams traverse a ring-configuration in opposite directions. Rotating the interferometer effectively shortens the optical path traveled by one of the beams, while lengthening the other. The interference pattern produced by the two beams displays a fringe shift proportional to the angular speed of rotation.
Ring laser gyros are now used extensively in inertial navigation systems for aircraft. Since RLGs require high vacuum and precision mirror technology, however, cost issues have limited their penetration of lower-end applications such as robotics, stabilization, and land navigation.
Fiberoptic gyros
By employing a coil of special-purpose optical fiber, FOGs perform the same tasks as RLGs at considerably less cost. The physical principal of operation is analogous to the Doppler effect, but in this instance it involves determination of the phase shift between the two counter-propagating light beams.
Interferometric fiber optic gyros can be configured as either closed-loop or open-loop, but the cost of the former presently restricts it to avionics and inertial navigation grade applications. The open-loop configuration consists of a fiber coil, one or two directional couplers, and a polarizer, all made from single-mode, polarization-maintaining optical fiber. In addition, the system requires an optical source and detector, and a piezoelectric (PZT) device wound with a small length of one end of the fiber coil to apply a non-reciprocal phase modulation.
