Ring Laser Gyroscopes Micah Larson April 21, 2005
Why use an RLG (Sagnac interferometer)? Demand for higher resolution in inertial navigation (commercial and military) Frictionless and therefore do not resist changes in orientation unlike their mechanical predecessors Small (relative to scope of application) as well as durable and accurate
Historical Background 1910’s: Sagnac experiments characterize rotational effects on a counter propagating beams of light 1960’s: Sperry develops first commercial RLG 1970’s: Honeywell develops first dithered RLG 1980’s: RLG’s begin to replace mechanical gyros in inertial navigation
Basic Composition Equilateral triangular glass block Three mirrors One allows 1% transmissionOne allows 1% transmission Mirrors are manufactured such that there is 1ppm lossMirrors are manufactured such that there is 1ppm loss He-Ne laser fill gas Laser source (anode- cathode current drive) Readout sensor
Basic Operation Two counter propagating (clockwise and counter-clockwise) beams travel around laser cavity While undergoing rotation, a fringe interference pattern is detected at the readout sensor Sagnac Effect
Sagnac Frequency Splitting Two rotating beams undergo frequency shifts, the corotating beam is seen as more red, counterrotating beam seen as more blue Frequency splitting coupled with the time and phase difference previously described determine RLG resolution Larger gyroscope = greater resolution
Lock-In When experiencing low angular rotation rates, it becomes possible for multiple gyros to “Lock-in” to the same frequency independent of angular velocity Eliminates the effects of Sagnac frequency splitting Similar to phenomenon observed in 1600’s by Christiaan Huygens in the behavior of pendulum clocks Gyros lock in is due to optical coupling from backscatter as opposed to mechanical couplingGyros lock in is due to optical coupling from backscatter as opposed to mechanical coupling Dithering at a higher frequency avoids lock in
Applications Inertial navigation Control systems Large scale applications (Large RLGs) Measuring variations in the angular rotation rate of EarthMeasuring variations in the angular rotation rate of Earth