Abstract
The suspended test masses of gravitational-wave (GW) detectors require precise alignment to be able to operate the detector stably and with high sensitivity. This includes the continuous counter-acting of seismic disturbances, which, below a few Hertz, are not sufficiently reduced by the seismic isolation system. The residual angular motion of suspended test masses is further suppressed by the Angular Sensing and Control (ASC) system. However, in doing so, the angular motion can be enhanced by the ASC at higher frequencies where the seismic isolation system is very effective. This has led to sensitivity limitations between about 10 Hz and 25 Hz of the LIGO detectors in past observation runs. The observed ASC noise was larger than simple models predict, which means that more accurate detector models and new simulation tools are required. In this article, we present Lightsaber, a new time-domain simulator of the ASC in LIGO. Lightsaber is a nonlinear simulation of the optomechanical system consisting of the high-power cavity laser beam and the last two stages of suspension in LIGO including the ASC. The main noise inputs are power fluctuations of the laser beam at the input of the arm cavities, read-out noise of sensors used for the ASC, displacement noise from the suspension platforms, and noise introduced by the suspension damping loops. While the plant simulation uses local degrees of freedom of individual suspension systems, the control is applied on a global angular basis, which requires a conversion between the local and global bases for sensing and actuation. Some of the studies that can be done with this simulation concern mis-centering of the beam-spot (BS) position on the test masses, the role of laser power fluctuations for angular dynamics, and the role of the various nonlinear dynamics. The next important step following this work will be a detailed comparison between Lightsaber results and data from the control channels of the LIGO detectors.
Highlights
Lightsaber is a nonlinear simulation of the optomechanical system consisting of the high-power cavity laser beam and the last two stages of suspension in LIGO including the ASC
The seismic isolation system of Advanced LIGO detectors consists of an active stage providing a low-vibration platform, from which a passive isolation system is suspended in the form of a quadruple pendulum stage (QUAD) [1,2]
The mechanical system is simulated in its local degrees of freedom, while the control is produced with respect to the global angular modes
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. As long as the DARM noise due to this coupling is well below the desired displacement sensitivity, and as long as the overall angular motion is sufficiently small so that the interferometer can be operated stably, the ASC performs well. The issue is that in the process of controlling TMs at low frequencies, high-frequency noise is introduced in the observation band originating mostly from the readout noise of sensors and less by imperfections of actuators at the penultimate mass (PUM) of the QUAD [5,6,17] This noise interferes directly with GW measurements. We present a time-domain simulator of the ASC It incorporates the dominant nonlinear couplings of the optomechanical system consisting of the high-power cavity laser beam and the last two stages of suspension in LIGO with the control system.
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