Ring lasers for geodesy

A rectangular 3.5 m perimeter prototype was constructed and put into operation in 1991. Here, we report on the improved performance after the device was located 30 m underground in Christchurch, New Zealand. In runs lasting well over one day, the Earth rotation produces linewidths down to 1 μHz; the rotation rate and associated phenomena are then resolved to about 100 nHz/70 Hz ≅1.4×10^-9, compatible with the estimated quantum noise of the 633 nm HeNe laser amplifier for large acquisition times of the order of 10⁵ s to 10⁶ s. To achieve this performance, however, drift of the Earth´s Sagnac frequency of the order of 6 mHz/2 h is removed. Pulling due to the finite lock-in frequency (at present ⩾10 Hz) has been studied. The unpulled frequency can be retrieved from the data through an analysis of the instantaneous frequency, without having to resort to optimization of pulling. The response of the prototype in short term (minutes) is tested by seismic events. Extrapolation of the performance to much larger rings appears secure. A plane square ring of 1 m² and a larger 16 m² device is planned to reach the regime of Earth rotation fluctuations