FIBER OPTIC GYROSCOPE (FOG) TECHNOLOGY

Introduction: The fiber optic gyroscope (FOG) is a solid state rotation sensor with no moving parts.  The rotation sensing part of aFOG consists of a very simple interferometer.  The interferometer includes a beam splitter and a single length of fiber formed into a loop.  The beam splitter splits a single input light wave into two light waves.  The two light waves are inserted into opposite ends of the fiber.  The two waves propagate through each other and arrive back at the beam splitter and are combined.  The optical power of the combined waves is then converted to an electrical signal and used to measure rate of rotation perpendicular to the loop of fiber.  The sensitivity to rotation is increased by increasing the size of the fiber loop.  Low-loss optical fiber provides an opportunity to increase the FOG sensitivity in a reasonably sized package by winding the fiber into a coil.  In this way we add sensitivity with each turn of fiber.

Sagnac Effect:  The fundamental effect upon which a FOG is based was demonstrated by Georges Sagnac in 1913. Sagnac set about to disprove the theory of relativity, and to support his argument he built an interferometer that now bears his name. The original Sagnac interferometer used mirrors and a beamsplitter. Such an interferometer (enclosing about one square mile) was used to measure the rotation rate of the Earth in 1925 to support the theory of relativity. Today a FOG can be made with an optical fiber and a directional coupler. Light is split by the directional coupler, enters the two ends of a single optical fiber formed into a coil, passes through itself; arrives back at the directional coupler and is combined as it emerges from the interferometer. In the non-rotating example (below left), the portion of light depicted as blue travels exactly the same path as the portion of light depicted as red and the two waves emerge from the interferometer at precisely the same time, i.e. in phase. In the rotating example (below right) the portion of light depicted as blue travels a longer path than the portion of light depicted as red and the blue light portion emerges delayed relative to the red light portion. The emerging light waves are not in phase. This phase difference is proportional to the rotation rate and is manifest as a change in optical power at the detector. For more information click here.

Sagnac_Effect-01.png

The figure to the left displays the principle of Sagnac interferometer. Source light splits to propagate in opposite directions through a fiber loop, passes through itself, and re-emerges after being combined. While the split light portions emerge at precisely the same time in the non-rotating case (left), the blue light travels further than the red light portion in the rotating example (right). The resulting phase difference is proportional to the rotation rate.

Multi-axis devices:  Each FOG measures a single axis of rotation.  Three FOGs are required to completely determine the three dimensional rotation rate vector. A three dimensional rotation rate vector is measured in an inertial measurement unit (IMU), an attitude and heading reference system (AHRS), an inertial navigation system (INS), and many gyrocompasses.