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RING LASER GYRO
The ring laser gyro uses laser light to measure angular rotation. Each gyro is a triangular-shaped, helium-neon laser that produces two light beams, one traveling in the clockwise direction and one in the counterclockwise direction. Production of the light beams, or lasing, occurs in the gas discharge region by ionizing the low pressure mixture of helium-neon gas with high voltage to produce a glow discharge. Light produced from the lasing is reflected around the triangle by mirrors at each corner of the triangle to produce the clockwise and counterclockwise light beams.
The path length around the cavity is carefully monitored and adjusted so that it is an integral multiple of the peak power laser wavelength.
When the laser beam frequencies differ, a fringe pattern of alternate dark and light stripes is created. Photodiodes sense the fringe pattern rate and direction of movement. The frequency and relative phase of the two diode outputs indicate magnitude and the direction of the gyro’s rotation.
At low rotation rates, the small frequency difference between the laser beams leads to beam coupling. This locks the frequencies together at a single false value. To compensate for this effect a peizoelectric dither motor is used to vibrate the laser block through the lock-in region. Dither vibration has a net zero average. It produces no net inertial rotation. The dither motor vibration can be felt on the IRU case and produces an audible hum.
Reference : Boeing Aircraft Maintenance Manuals