K. Ulrich Schreiber

Correction of backscatter-induced systematic errors in ring laser gyroscopes.

In ring laser gyroscopes, backscatter coupling between the counterpropagating beams leads to systematic errors in measured rotation rates. We show the errors can be derived from the nearly independent optical frequency perturbations of the separate beams. In a process of double backscatter each beam is scattered first into the other beam direction, then rescattered back into itself, giving phase and amplitude perturbations. The analysis proceeds from a purely passive ring cavity, through inclusion of a gain medium and gain saturation. The rotation rate errors may be estimated and corrected from the modulations of the counterpropagating beams. The method is demonstrated with real data.

AUTOREGRESSIVE ANALYSIS FOR THE DETECTION OF EARTHQUAKES WITH A RING LASER GYROSCOPE

The second-order autoregressive AR(2) model is used to analyze rotational data for seismic events captured by a large ring laser gyroscope. Both the Sagnac frequency and linewidth estimates obtained from this model sense the rotational components of seismic waves. An event of magnitude ML = 6.5 at a distance of D = 5.4° from a large ring laser gyroscope operating at its quantum limit is used to compare the AR(2) model with the previous analytical phase angle method of analysis. The frequency, linewidth and analytic phase angle data each satisfactorily estimate the rotation magnitude. The direct detection of rotational motion in the P wave coda is observed, demonstrating the conversion to transverse S wave polarizations by the local geology.

Large ring laser gyroscopes: towards absolute rotation rate sensing

Ring laser gyroscopes have increased in sensitivity by six orders of magnitude over the last several decades such that they are poised to make valuable contributions to geodesy and terrestrial tests of general relativity. To fully exploit their capabilities, residual (time varying) read out errors arising from backscatter coupling must be physically minimized or otherwise compensated. We present the results of a backscatter correction process for a 12.25 m2 gyroscope with a vast improvement in long term rotational sensitivity.

Satellite laser ranging in the near-infrared regime

Satellite Laser Ranging Systems typically operate on the second harmonic wavelength of a pulsed Nd:YAG laser at a wavelength of 532 nm. The absence of sufficiently sensitive photo-detectors with a reasonably large active area made it beneficial to trade the conversion loss of frequency doubling against the higher quantum efficiency of the detectors. Solid state silicon detectors in the near infra-red regime at λ = 1.064 µm also suffered from high thermal noise and slow signal rise times, which increased the scatter of the measurements by more than a factor of 3 over the operation at λ = 532 nm. With the availability of InGaAs/InP compound - Single Photon Avalanche Diodes the situation has changed considerably. Their quantum efficiency has reached 70% and the compound material of these diodes provides a response bandwidth, which is commensurate with high high speed detectors in the regime of 532 nm. We have investigated the properties of such a diode type Princeton Lightwave PGA-200-1064 for its suitability for SLR at the Nd:YAG fundamental wavelength with respect to the quantum efficiency and their timing properties. The results are presented in this paper. Furthermore, we provide remarks to on the performance of the diode compared to state of the art detectors, that operate at the Nd:YAG second harmonic wavelength. Finally, we give an estimate of the photoelectron statistics in satellite laser ranging for different operational parameters of the Wettzell Laser Ranging System.

Crystalline coatings for near-IR ring laser gyroscopes

Substrate-transferred crystalline coatings represent an entirely new concept in high-performance optical coatings. This technology was originally developed as a solution to the long-standing thermal noise limitation found in ultrastable optical interferometers, impacting cavity-stabilized laser systems for precision spectroscopy and optical atomic clocks, as well as interferometric gravitational wave (GW) detectors [1]. The ultimate stability of these systems is currently dictated by coating Brownian noise, driven by the excess mechanical losses of the materials that comprise the highly reflective elements of the cavity end mirrors. Compared with state-of-the art ion-beam sputtered dielectric reflectors, crystalline coatings, comprising substrate-transferred GaAs/AlGaAs heterostructures, exhibit competitive reflectivity together with a significantly enhanced mechanical quality, resulting in a thermally-limited noise floor consistent with a tenfold reduction in mechanical damping at room temperature [2]. Building upon this initial demonstration, we have recently developed high-performance crystalline supermirrors with parts-per-million levels of optical losses, including both absorption and scatter, at wavelengths spanning 1000 to nearly 4000 nm, with experimentally verified absorption coefficients below 0.1 cm-1 in the near infrared [3]. These advancements have opened up additional application areas including the focus of this work. Here we demonstrate the first implementation of crystalline supermirrors in an active laser system, expanding the core application area of these low-thermal noise cavity end mirrors to inertial sensing systems and specifically next-generation high-sensitivity ring-laser gyroscopes [4,5].

Calibration of system delays in the European Laser Timing to 10 ps accuracy

Recently the European Laser Timing experiment is under construction. It is an optical link prepared in the frame of the European Space Agency mission Atomic Clock Ensemble in Space. The objective of this laser time transfer is the synchronization of the ground based clocks and the clock on board the International Space Station with precision of the order of units of picoseconds and the accuracy of 50 ps. We are reporting on a progress and results in calibration of the system delays involved.