Omega-E: Establishing the instantaneous Earth rotation vector by inertial rotation sensing
Final Report Abstract
The ring laser structure ROMY – ‘Rotational Motion in seismologY’ was built under design rules of seismological and later extension for geodetical application by the fund of LMU (Ludwigs-Maximilians Universität, Department für Geo- und Umweltwissenschaften), at the Geophysical Observatory Fürstenfeldbruck. Based on the GeoSensor design, it was constructed in a tetrahedron shape. Its whole structure stands on one tip and each surface is defined by a triangular ring laser having a 12 m side length, deriving the inertial rotation vector measured locally on the Earth’s surface in the topocentric as well as in the geocentric reference frame. The best obtained sensitivity for a component is about 1 prad/s. An uncontrolled / un-stabilized ring laser is called free running. Since all four components of ROMY are free running, the application of the instrument is currently limited to seismological observation. Due to the instability of the operational environment such as temperature and pressure fluctuations, the ring laser perimeter is affected. Once the optical path-length changes for more than 0.6 µm, modehops would occur. When the two counter-propagating longitudinal modes (single-mode operation) both jump to the same mode index, the measured Sagnac frequency changes in a small amount. However, ROMY cannot function as a rotation sensor when it is in splitmode operation, as the frequency difference (at about 8.3 MHz) between two operational modes is too large to detect an optical beat. Firstly, each component of ROMY adopts a base control unit functioning as ‘health’ monitor and control. A duty cycle of more than 98% is achieved once all mechanical components are optimally aligned. The actual status is stored in an archive, presented on a website, and delivered daily to dedicated recipients. This tool is very useful to evaluate the actual operating conditions of each ring laser component. It also provides the information regarding alignment status of the supermirrors, gain tube, and optical elements. Therefore, the need for instrument maintenance can be noticed in time. A closed feedback control loop for the perimeter stabilization was applied to achieve better long-term operation on a consistent mode number. Initially, a self-referenced perimeter control technique was employed. However, this was found inefficient as the feedback signal was frequently perturbed by the laser dynamics. Another method was investigated with promising deliverables. Due to the pandemic situation since the beginning of 2020, it was not possible to completely finish this task. For the ROMY ring laser, an intense and regular maintenance of each cavity component is required in order to achieve long-term rotation sensing for continuous derivation of the Earth rotation axis. This was the first time for a ground based rotational sensor to estimate the position of the Earth rotation axis in global space.
Publications
- On the Influence of Diurnal and Subdiurnal Signals in the Normal Vector on Large Ring Laser Gyroscope Observations, Pure Appl. Geophys. Vol. 177, pp. 4217–4228
Tercjak, M., Gebauer, A., Rajner, M., Brzeziński, A., Schreiber, K. U.
(See online at https://doi.org/10.1007/s00024-020-02484-2) - Reconstruction of the Instantaneous Earth Rotation Vector with Sub-Arcsecond Resolution Using a Large Scale Ring Laser Array, Phys. Rev. Lett., vol. 125 (3), pp. 33605–33610
Gebauer, A, Tercjak, M., Schreiber, K. U., Heiner, I., Kodet, J., Hugentobler, U., Wassermann, J., Bernauer, F., Lin, C.-J., Donner, S., Egdorf, S., Simonelli, A., Wells, J.-P. R.
(See online at https://doi.org/10.1103/physrevlett.125.033605) - Gyroscopic performance and some seismic measurements made with a 10 meter perimeter ring laser gyro housed in the Ernest Rutherford building, Appl. Opt., vol. 60, pp. 1737-1743
Zou D., Thirkettle, R. J., Gebauer, A., MacDonald, G. K., Schreiber, K. U., Wells, J.-P. R.
(See online at https://doi.org/10.1364/ao.414897)
