Finding your way with FOGs

For those who are directionally impaired, a FOG may be in your future. In fact, one already may be helping you find your way. With a fiber-optic gyroscope (FOG) in place, the navigation system of some automobiles remains accurate even when the GPS signal they depend on is blocked or otherwise unavailable. Other, higher-performance FOGs serve in military and space applications. FOGs have several advantages over traditional, mechanical gyroscopes: With zero moving parts, they are inexpensive, easy to manufacture, and require minimal maintenance.

Figure 1 Fiber-optic-gyroscope rotation causes a phase shift in light beams traveling a ring path in opposite directions.

The basic principle underlying FOG operation, now known as the Sagnac effect, was published in 1913 by G. Sagnac. The effect describes a beam of light split in two, with each beam traveling the same ring path but in opposite directions. As long as the path doesn't rotate, the beams exit the path in phase. Any rotation, however, causes the light going in one direction to travel a different distance from light going in the opposite direction. During the time it takes light to transit the path, the exit point of the path will have moved, one beam will have traveled a shorter path than the other (Figure 1), and the two beams will be out of phase when exiting the path. This phase shift can be measured to determine the rotation of the ring.

FOGs from Fibersense Technology (Canton, MA; www.nsd.es.northropgrumman.com) incorporate five basic elements (Figure 2): a solid-state light source, an

Figure 2 An integrated optic circuit (IOC) and solid-state light source deliver dual light beams to a FOG coil.

A FOG design must balance several application-specific considerations:

• Longer fiber coils increase the light transit time, thereby increasing the time period for the phase difference to develop and increasing the sensitivity and signal-to-noise ratio.
• A larger diameter spool results in greater velocity of the coil at its periphery for a given angular rotation, again increasing the phase difference between the two light beams, similarly improving sensitivity and signal-to-noise ratio. Larger spools, however, are more prone to thermal and vibration disturbances.
• Shorter light-beam wavelengths increase sensitivity, but longer wavelengths yield better radiation resistance for space applications.

Typical FOGs from Fibersense contain from 100 to 1000 m of fiber on spools ranging from 20 to 100 mm in diameter and employ 850-, 1330-, and 1550-nm wavelengths. They come in one-, two-, and three-dimensional configurations for applications ranging from vehicle navigation to camera platform stabilization to precise satellite guidance systems.