RING LASERS FOR GEODESY

A rectangular 3.5 m perimeter prototype was constructed and put into o peration in 1991. Here, we report on the improved performance after th e device was located 30 m underground in Christchurch, New Zealand. In runs lasting well over one day, the earth rotation produces linewidth s down to 1 mu Hz; the rotation rate and associated phenomena are then resolved to about 100 nHz/70 Hz congruent to 1.4x10(-9), compatible w ith the estimated quantum noise of the 633 nm HeNe laser amplifier for large acquisition times of the order of 10(5) s to 10(6) s. To achiev e 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 f requency (at present greater than or equal to 10 Hz) has been studied. The unpulled frequency can be retrieved from the data through an anal ysis of the instantaneous frequency, without having to resort to optim ization of pulling. The response of the prototype in short term (minut es) is tested by seismic events. Extrapolation of the performance to m uch larger rings appears secure. A plane square ring of 1 m(2) and a l arger 16 m(2) device is planned to reach the regime of earth rotation fluctuations.