Multioscillator laser gyros

The two-mode ring laser gyro is reviewed. Standard bias and dither techniques are discussed that alleviate the mode locking encountered at low rotation rates. The four-mode DILAG (differential laser gyro) and its magnetic field extension the ZLAG (Zeeman laser gyro) are defined and analyzed. These devices contain two coresident ring lasers of opposite helicity and may have no moving parts. The mode-locking problems are overcome with the use of various biases that cancel out in the rate signal. A semiclassical analysis is reviewed that gives the electric field amplitudes and frequencies of the four modes in the presence of multilevel atoms, cavity anisotropy, and backscattering. It is shown that without mode-locking contributions, laser operation can be approximated very well by four independently oscillating modes. The Zeeman effects are considered. Novel scalar four-mode schemes are also discussed that circumvent the mode-locking problem.