ringlasergyro

A ring laser gyro (RLG), also referred to as a ringlasergyro in some texts, is an optical gyroscope that measures angular velocity by exploiting the Sagnac effect in a closed-path laser cavity. It provides a direct readout of rotation rate without relying on mechanical torsion or moving parts in the sense of rotating proof masses.

Two laser beams circulate in opposite directions around a closed loop; when the apparatus rotates, the path length differs for the two beams, creating a beat frequency between the counter-propagating beams proportional to the rotation rate. The beat frequency is detected by a photodetector and converted to angular velocity by calibration.

The cavity is typically a rigid triangular or square loop made from low-thermal-expansion material, with a laser source, typically a helium-neon or diode-pumped solid-state laser, and mirrors forming the resonator. Active controls stabilize the optical frequencies and suppress mode hops; the interfered output is monitored by a photodetector and signal-processing electronics.

RLGs offer high bandwidth and robust performance, but are subject to noise sources such as shot noise, backscatter, and thermal drift. At very low rotation rates, mode coupling (lock-in) can suppress the signal, which is mitigated by dithering or using multiple cavities.

They are widely used in inertial navigation systems for aircraft, submarines, spacecraft, missiles, and guided weapons, often as part of an integrated inertial measurement unit with accelerometers. They provide continuous, drift-free angular rate information and can have long-term stability without external references.

The concept originated in the 1960s with early ring laser experiments and matured during the 1970s–1980s with commercial implementations. Major defense and aerospace companies produced RLGs, and they have largely been complemented by fiber optic gyros in some platforms.