A six degree-of-freedom fused silica seismometer: design and tests of a metal prototype

Abstract

Ground vibrations couple to the longitudinal and angular motion of the aLIGO test masses and limit the observatory sensitivity below 30 Hz. Novel inertial sensors have the potential to improve the aLIGO sensitivity in this band and simplify the lock acquisition of the detectors. In this paper, we experimentally study a compact 6D seismometer that consists of a mass suspended by a single wire. The position of the mass is interferometrically read out relative to the platform that supports the seismometer. We present the experimental results, discuss limitations of our metallic prototype, and show that a compact 6D seismometer made out of fused silica and suspended with a fused silica fibre has the potential to improve the aLIGO low frequency noise.

Compact Michelson interferometers with subpicometer sensitivity

“…Compact, interferometric position sensors are well positioned as candidates for the next generation of displacement readout in precision applications. Two such examples, the 6D seismometer [25] and the Advanced LIGO quadruple suspensions [17], can benefit from compact interferometers. The current sensors have sufficient range but their current level of sensitivity results in the injection of excess noise below 30 Hz.…”

“…2a. In the unconstrained mirror application, such as in the readout of a multiple-degree-of-freedom seismometer [25], this factor will be of particular importance as the seismic measurement will become dominant over other noise sources and will itself be down-converted and potentially obscure the measurement at other frequencies. We are interested in enhancing the precision towards the low frequency limit and, fortunately, the ground motion inherently decreases towards the high frequencies.…”

Optimal readout scheme for multi-degree-of-freedom seismometers

“…Recently, a six-dimensional seismometer has been proposed as an alternative approach to tackle the horizontal-tilt coupling [6][7][8]. In [7], the authors, including some of us, reported on a compact metal prototype to demonstrate the basic principle.…”

“…Recently, a six-dimensional seismometer has been proposed as an alternative approach to tackle the horizontal-tilt coupling [6][7][8]. In [7], the authors, including some of us, reported on a compact metal prototype to demonstrate the basic principle. They analyzed the coupled horizontal-tilt dynamics by measuring and modeling the transfer functions, and also identified the main noise sources, which include the optical readout noise and the thermal noise.…”

“…Therefore only if there were no correlation, κ would vanish and directly inverting the sensing matrix would coincide with the optimal estimate. Using the same set of parameter values in [7] for the fused silca 6D seismometer, the optimal sensitivity for the horizontal degree of freedom is shown in figure 1. To illustrate the ultimate performance of the optimal readout scheme, in this plot we have ignored the tilt noise from the ground itself S gnd θ0θ0 , which depends on the specific environmental condition.…”