Sagnac Sensor
Inertial sensors utilizing the Sagnac effect detect angular rotation by measuring the frequency difference between counter-propagating laser beams in a closed optical cavity. A ring laser gyro consists of a solid glass block with drilled channels that form a triangular or square path for the light. The technology provides high stability and low bias drift compared to mechanical or micro-scale alternatives.
Laser Cavity
Helium-neon gas inside the channels acts as the gain medium when excited by a high voltage discharge. The light travels in both directions simultaneously, and the two beams interfere at a readout detector. When the ring laser gyro rotates, one path length effectively shortens while the other lengthens, creating a beat frequency proportional to the rotation rate.
Digital output from this process is inherently linear and does not require complex analog to digital conversion. Mirrors at the corners of the cavity must be polished to extreme tolerances to minimize scattering and maintain the quality of the laser.
Lockin Effect
At very low rotation rates, the two counter-propagating beams can synchronize and oscillate at the same frequency due to backscatter from the mirrors. The lockin effect creates a dead zone where the ring laser gyro fails to report any rotation. To overcome this, the entire block is often mechanically dithered or vibrated at a known frequency to keep the sensor in a dynamic range where it remains responsive.
Long Reliability
Depletion of the gas mixture or the degradation of the cathode typically limits the long-term reliability of the unit. Sealed housings and high purity materials extend the service life to tens of thousands of hours in demanding aerospace applications.
