Ring Laser Gyroscopes

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Ring Laser Gyroscopes

• Micah Larson
• April 21, 2005

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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

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Historical Background

• 1910s Sagnac experiments characterize
  rotational effects on a counter propagating beams
  of light
• 1960s Sperry develops first commercial RLG
• 1970s Honeywell develops first dithered RLG
• 1980s RLGs begin to replace mechanical gyros
  in inertial navigation

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Basic Composition

• Equilateral triangular glass block
• Three mirrors
• One allows 1 transmission
• Mirrors are manufactured such that there is 1ppm
  loss
• He-Ne laser fill gas
• Laser source (anode-cathode current drive)
• Readout sensor

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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

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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

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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 1600s by
  Christiaan Huygens in the behavior of pendulum
  clocks
• Gyros lock in is due to optical coupling from
  backscatter as opposed to mechanical coupling
• Dithering at a higher frequency avoids lock in

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Applications

• Inertial navigation
• Control systems
• Large scale applications (Large RLGs)
• Measuring variations in the angular rotation rate
  of Earth