In order to keep something
stable, you need a stability
reference. We get our stability reference from
little liquid-filled tubes in our
inner ears, each lined with thousands of delicate
microscopic hairs that sense any movement in any direction. A
gyroscope provides the perfect
artificial "inner ear" for
mechanical devices.
In pursuit of the best performance possible in a gyroscope for high-precision applications like those of navigation, guidance, and control in aerospace and the military, more precise gyroscopes are needed than can be created with a spinning wheel. In addition, at increasing acceleration and shock levels, systems with spinning mechanical elements are difficult to operate and maintain, and have a high hysteresis.
Enter the Laser gyroscope, which consists of a series of mirrors bouncing a laser beam around. Any microscopic movement of the gyroscope is reflected (no pun intended) in the laser beams, whose absolute position and frequency change depending on the nature of the motion. However, these devices are not cheap to make rugged enough to survive in hazardous environments.
Enter the Fiber-Optic Gyroscope (FOG). Instead of a series of mirrors, a FOG uses a coil of optical fiber for each axis of motion. When light goes through a rotating fiber coil (the rotation provided by the object the coil is fastened to), the time the beam takes depends on the rotation rate of the coil. When beams are shot through the coil from each end, this creates a phase difference between the beams, which is measured to determine the rate of rotation. An advantage of using fiber over mirrors is that the stack of fiber is inherently more rugged as no special alignment is neccessary. One can almost say the loops of coiled fiber are an optoelectronic version of the loops of tubes in our inner ear, performing the exact function in an (eerily?) similar way.
This technology is being strongly challenged by MEMS-based devices using vibrating microscopic structures on a silicon chip.