Plane of Precession Plane of Force Plane of Rotation Applied force Resulting motion (precession) Applied force B e a m 1 B e a m 2 Ω Start and finish—nonrotationg path Start Finish when path rotates Direction of  path rotation Frequency  difference A ring laser gyro functions due to the Sagnac Effect Figure 10-96.  Light traveling in opposite directions around a non- rotating path arrives at the end of the loop at the same time (top). When the path rotates, light traveling with the rotation must travel  farther to complete one loop. Light traveling against the rotation  completes the loop sooner (bottom). Figure 10-95.  When a force is applied to a spinning gyroscope, it  reacts as though the force came from 90° further around the rotor in  the direction it is spinning. The plane of the applied force, the plane  of the rotation, and the plane in which the gyro responds (known  as the plane of precession), are all perpendicular to each other.  Ring Laser Gyros (RLG) 10-55 RLGs are very rugged and have a long service life with  virtually no maintenance due to their lack of moving parts.  They measure movement about an axis extremely quickly and  provide continuous output. They are extremely accurate and  generally are considered superior to mechanical gyroscopes.  phase shift detected between the counterrotating lasers is  proportional to the rate that the aircraft is moving about that  axis. On aircraft, an RLG is installed for each axis of flight.  Output can be used in analog instrumentation and autopilot  systems. It is also easily made compatible for use by digital  display computers and for digital autopilot computers.  An RLG is remotely mounted so the cavity path rotates  around one of the axes of flight. The rate of frequency  A laser is light amplification by stimulated emission of  radiation. A laser operates by exciting atoms in plasma to  release electromagnetic energy, or photons. A ring laser  gyro produces laser beams that travel in opposite directions  around a closed triangular cavity. The wavelength of the  light traveling around the loop is fixed. As the loop rotates,  the path the lasers must travel lengthens or shortens. The  light wavelengths compress or expand to complete travel  around the loop as the loop changes its effective length.  As the wavelengths change, the frequencies also change.  By examining the difference in the frequencies of the two  counterrotating beams of light, the rate at which the path is  rotating can be measured. A piezoelectric dithering motor in  the center of the unit vibrates to prevent lock-in of the output  signal at low rotational speeds. It causes units installed on  aircraft to hum when operating.  [Figure 10-97]  The ring laser gyro (RLG) is widely used in commercial  aviation. The basis for RLG operation is that it takes time  for light to travel around a stationary, nonrotating circular  path. Light takes longer to complete the journey if the path is  rotating in the same direction as the light is traveling. And, it  takes less time for the light to complete the loop if the path is  rotating in the direction opposite to that of the light. Essentially,  the path is made longer or shorter by the rotation of the path.  [Figure 10-96]  This is known as the Sagnac effect.