Enhanced ring lasers: a new measurement tool for Earth sciences

We report the progress in the technology of fabrication of large ring lasers that has resulted in an increase in instrumental rotation sensitivity by as much as a factor of 3, to δΩ = 1.2 × 10^-11 rad s^-1 Hz^-1/2, which makes the domain of changes in the angular velocity of Earth's rotation, ΔΩ/Ω ≈ 10^-9, accessible to a local rotation sensor. New studies show that the largest contribution to the observed deviation in sensor performance with respect to the computed shot noise limit is caused by the micro-seismic background activity of the Earth. Our efforts have been concentrated on the improvement of sensor stability, including correction of drift effects, which are caused by the aging of the laser gas, fixing scale factor instabilities induced by atmospheric pressure variations, and minimising the temperature variations resulting from corresponding adiabatic expansion and compression of the local air around the instrument. To achieve this, we have recently introduced a pressure-stabilising vessel with dimensions slightly larger than the ring laser apparatus, such that it encloses the entire structure. By monitoring the optical frequency in the ring laser cavity continuously and stabilising the scale factor in a closed loop system with the pressure-stabilising vessel, it has become possible to extend the range of sensor stability from the short term (1 — 3 days) to well into the mid-term regime (>40 days), and possibly even well beyond that. Once a sufficiently long timeseries of the ring laser data has been recorded, we will be able to define the range of temporal stability in more detail. The extension of the regime of stability gives access to geophysical signals at frequencies substantially lower than previously observable with ring lasers.