Enhanced G: Precision Ring Laser Earth Rotation Sensing for Space Geodesy at the Quantum Limit
Final Report Abstract
The ring laser project on the Geodetic Observatory Wettzell investigates the suitability of the technique of inertial rotation sensing for applications in space geodesy. This requires a sensor resolution of 1 part in 10^9 for the measurement of Earth rotation and a long-term stability of the ring laser structure over many weeks. While the sensitivity of the G ring provides a resolution of 12 pico-rad/s, we find that the stability exhibits a tiny and variable drift after about one day. Most of this effect is caused by a small amount of backscatter coupling, which is variable in time. In this research project we have tried to remove the influence of backscatter coupling in two ways: a) Operating the ring laser on different longitudinal mode indices for the two counterpropagating laser beams. b) Monitor the side length of at least one side of the square ring laser structure in order to get access to a measurement quantity, which may eventually describe the variation of the backscatter phase. It turned out that it was well possible to operate the 16m perimeter G ring laser on two different longitudinal mode indices reliably. It required a very stable frequency source (hydrogen maser) to subtract frequency offset of one free spectral range from the Sagnac interferogram. However this mode of operation did not provide any advantages. As opposed to the usual operation on the same longitudinal mode index cavity related effects do not cancel out and it was not possible to control such a large cavity well enough. As reported it was found that backscatter coupling cannot be reduced by offsetting the two optical beams further away from each other. By directly measuring the amount of backscatter coupling and modeling these effects we have achieved the desired improvement. The monitoring of the variation of the length of one side of the ring laser did not provide the expected resolution. Since the interferogram also contained variations from the differential beam path outside of the cavity, we could not infer the variation of the side length well enough. To compensate this problem, we have developed a self-referencing approach to stabilize the ring laser cavity. It turned out that the ring laser can be operated on more than one laser mode per sense of propagation. Therefore it was possible to stabilize the ring laser on the beat note from the laser frequencies spaced apart by 6 free spectral ranges (112.4 MHz). This mode of operation combined with the new backscatter model improved the quality of the G ring laser measurements substantially. The objective of the third work package was the development of a program suite that allows a consistent data analysis of the ring laser time series of rotation measurements. It contains a number of geophysical models that have influence of the measurements of a ring laser, rigidly attached to the Earth at a given location and under an arbitrary orientation. The program allows to screen and select the rotation data as well as jointly analyze any ancillary observation time series (such as local tilt), that may have influence on the actual value of the measured rotation rate as a function of time. One of the several output options is the output as a SINEX (Solution INdependent EXchange) format. This makes the ring laser observations generally available to the space geodesy community.
Publications
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(2017) High-accuracy absolute rotation rate measurements with a large ring laser gyro: establishing the scale factor. Applied optics 56 (4) 1124–1130
Hurst, Robert B.; Mayerbacher, Marinus; Gebauer, Andre; Schreiber, K. Ulrich; Wells, Jon-Paul R.
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Closed-loop locking of an optical frequency comb to a large ring laser, Optics Letters, vol. 38, no. 18, pp. 3574–3577
Schreiber, K. U.; Gebauer, A.; Wells, J.-P. R.
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The centennial of the Sagnac experiment in the optical regime: From a tabletop experiment to the variation of the Earth's rotation, Comptes Rendus Physique, vol. 15, no. 1, pp. 859–865, Dec. 2014
Schreiber, K. U.; Gebauer, A.; Igel, H.; Wassermann, J.; Hurst, R. B.; Wells, J.-P. R.
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Estimate of Rayleight-to-Love wave ratio in the secondary microseism by colocated ring laser and seismograph; Geophysical Research Letters
Tanimoto, T.; Hadziioannou, C.; Igel, H.; Wassermann, J.; Schreiber, U.; Gebauer, A.
