A. Gebauer

Estimate of Rayleigh‐to‐Love wave ratio in the secondary microseism by colocated ring laser and seismograph

©2015. American Geophysical Union. All Rights Reserved. Using a colocated ring laser and an STS-2 seismograph, we estimate the ratio of Rayleigh-to-Love waves in the secondary microseism at Wettzell, Germany, for frequencies between 0.13 and 0.30 Hz. Rayleigh wave surface acceleration was derived from the vertical component of STS-2, and Love wave surface acceleration was derived from the ring laser. Surface wave amplitudes are comparable; near the spectral peak about 0.22 Hz, Rayleigh wave amplitudes are about 20% higher than Love wave amplitudes, but outside this range, Love wave amplitudes become higher. In terms of the kinetic energy, Rayleigh wave energy is about 20-35% smaller on average than Love wave energy. The observed secondary microseism at Wettzell thus consists of comparable Rayleigh and Love waves but contributions from Love waves are larger. This is surprising as the only known excitation mechanism for the secondary microseism, described by Longuet-Higgins (1950), is equivalent to a vertical force and should mostly excite Rayleigh waves.

Long-term frequency stabilization of a 16 m 2 ring laser gyroscope

A 16 m(2) helium-neon-based ring laser gyroscope has been frequency stabilized to within 60 kHz over a period of three months. This is achieved using the beat frequency of the ring laser and an iodine-stabilized reference laser as a feedback signal on a pressure vessel enclosing the entire laser, under servo control. We demonstrate that we can compensate for, and thereby negate the influence of, atmospheric pressure variations, which are considerable sources of long-term instability.

Seasonal variations in the Rayleigh-to-Love wave ratio in the secondary microseism from colocated ring laser and seismograph

Monthly variations in the ratio of Rayleigh-to-Love waves in the secondary microseism are obtained from a colocated ring laser and an STS-2 seismograph at Wettzell, Germany. Two main conclusions are derived for the Rayleigh-to-Love wave kinetic energy ratios in the secondary microseism;first, the energy ratio is in the range 0.8-0.9 ( 1.0) throughout a year except for June and July. It means that Love wave energy is larger than Rayleigh wave energy most of the year by about 10-20%. Second, this ratio suddenly increases to 1.0-1.2 in June and July, indicating a larger fraction of Rayleigh wave energy. This change suggests that the locations and behaviors of excitation sources are different in these months.

Closed-loop locking of an optical frequency comb to a large ring laser

We report on the frequency locking of a 16  m2 ring laser to a single tooth of an optical frequency comb referenced to a hydrogen maser, obtaining a frequency stability of 1 kHz over several days. In common mode operation, where the counterpropagating laser beams run on the same longitudinal mode index, a sensitivity to rotation of 3×10(-9) relative to Earths rotation is obtained. To test a proposal to bypass time-varying backscatter-induced readout errors in large ring laser gyroscopes, we have operated the laser on adjacent longitudinal cavity modes. The Sagnac frequency due to Earths rotation obtained in this fashion was strongly influenced by atmospheric pressure changes because the counterpropagating beams within the cavity are affected differently by geometric cavity fluctuations.

A laser gyroscope system to detect the gravito-magnetic effect on Earth

Ring lasers are inertial sensors for angular velocity based on the Sagnac effect. In recent years they have reached a very high sensitivity and accuracy; the best performing one, the ring Laser G in Wettzell (Germany), a square ring with 16 m perimeter, has reached a sensitivity of 12prad/s very close to the shot noise limit inferred from ring-down time measurements. On this basis it is expected that an array of six square ring lasers of 36 m perimeter, can perform a 1% accuracy test for the measurement of the Lense-Thirring frame dragging after 2 years of integration time. Essential for this measurement is the comparison between the Earth angular velocity and orientation in space measured with the ring array and compared to the measurement series maintained by the International Earth Rotation and Reference System Service (IERS), which measures Earth Rotation and pole position with respect to remote quasars. It has been shown that the accuracy of G in Wettzell is limited by the low frequency motion of the near surface laboratory, which is of the order of several prad/s, roughly 100 times larger than the Lense-Thirring contribution. For this reason the entire experiment should be placed in a quite underground laboratory, where these perturbations are reduced. The feasibility to properly place such a device inside the GranSasso INFN National Laboratory has been investigated.

An Event Database for Rotational Seismology

We introduce a new event database for rotational seismology. The rotational seismology website (see Data and Resources) grants access to 17,000+ processed global earthquakes starting from 2007. For each event, it offers waveform and processed plots for the seismometer station at Wettzell and its vertical-component ring laser (G-Ring), as well as extensive metrics (e.g., peak amplitudes, signal-to-noise ratios). Tutorials and illustrated processing guidelines are available and ready to be applied to other data sets. The database strives to promote the use of joint rotational and translational ground-motion data demonstrating their potential for characterizing seismic wavefields.

High-Accuracy Ring Laser Gyroscopes: Earth Rotation Rate and Relativistic Effects

The Gross Ring G is a square ring laser gyroscope, built as a monolithic Zerodur structure with 4 m length on all sides. It has demonstrated that a large ring laser provides a sensitivity high enough to measure the rotational rate of the Earth with a high precision of ΔΩE < 10-8. It is possible to show that further improvement in accuracy could allow the observation of the metric frame dragging, produced by the Earth rotating mass (Lense-Thirring effect), as predicted by General Relativity. Furthermore, it can provide a local measurement of the Earth rotational rate with a sensitivity near to that provided by the international system IERS. The GINGER project is intending to take this level of sensitivity further and to improve the accuracy and the long-term stability. A monolithic structure similar to the G ring laser is not available for GINGER. Therefore the preliminary goal is the demonstration of the feasibility of a larger gyroscope structure, where the mechanical stability is obtained through an active control of the geometry. A prototype moderate size gyroscope (GP-2) has been set up in Pisa in order to test this active control of the ring geometry, while a second structure (GINGERino) has been installed inside the Gran Sasso underground laboratory in order to investigate the properties of a deep underground laboratory in view of an installation of a future GINGER apparatus. The preliminary data on these two latter instruments are presented.

Precision Cavity Control for the Stable Operation of a Large Ring Laser Gyroscope

Currently the sensor performance of large ring laser gyroscopes is limited more by stability in the long term rather than measurement resolution. This is mostly because of a variable contribution of backscatter coupling between the two counter-propagating laser beams inside the square ring laser cavity. Introducing an atmospheric pressure stabilizing vessel around the ring laser structure allows us to compensate variations in the compression of the ring laser body by ambient pressure changes. Adding an interferometric feedback system takes this approach one step further in that it allows us to stabilize the length of the cavity to be stable to within 1 kHz of optical frequency. However it transpires that this precision cavity control is not sufficient to keep the backscatter coupling sufficiently constant to reduce the variation of the offset bias of the gyroscope to values of 10 μHz or below. A tightly controlled perimeter does not preclude small variations of the 4 individual sides of the gyroscope. In the absence of sufficient control over the backscatter process or sufficiently precise numerical estimate of the backscatter variation, a tight control of the length of the four individual arms of the gyroscope, in addition to the precision perimeter control appears to be a viable experimental approach. Monitoring the phase relationship between the ring laser beat note, taken at different corners of the gyroscope, provides the necessary access to the desired cavity control. This paper reports the first results of this investigation.