ringlaser

A ring laser, or ring laser gyroscope, is a laser in which the optical resonator forms a closed loop, allowing light to circulate in two counterpropagating directions around the loop. The most common implementations use a gaseous laser gain medium such as helium–neon, arranged in a triangular or square ring, but solid-state and diode-pumped versions exist as well. The device is designed to measure rotation and is widely used as a high-precision inertial sensor.

In operation, the counterpropagating beams experience the Sagnac effect when the apparatus rotates. Rotation causes a difference in optical path length for the two directions, producing a small frequency (or phase) difference between the beams. The beams are combined to produce a beat frequency that is proportional to the angular rate. For a ring with area A, perimeter L, and light wavelength λ, the approximate beat frequency is Δf ≈ (4 A Ω) / (λ L), where Ω is the rotation rate. The scale factor, along with bias and noise, determines the sensor’s sensitivity and dynamic range.

Design notes: common ring geometries include triangular or square cavities; high-quality mirrors and precise alignment are required to maintain a stable single-frequency operation. At low rotation rates, backscattering can cause a lock-in (dead band), which is mitigated by dithering or electronic stabilization. Modern systems use active stabilization and error-correction schemes within inertial navigation assemblies.

Applications and history: ring lasers were developed for inertial navigation in aviation, ships, submarines, and spacecraft. They offer a solid-state alternative to mechanical gyros and have inspired related devices, including fiber-optic gyros and integrated photonic ring resonators.