How to reduce the suspension thermal noise in LIGO without improving the Q's of the pendulum and violin modes

TL;DR

The paper proposes methods to significantly reduce suspension thermal noise in LIGO by optimizing laser beam positioning and implementing a readout scheme to subtract top attachment noise, enhancing detector sensitivity without increasing Q factors.

Contribution

It introduces a novel approach to reduce suspension thermal noise in gravitational wave detectors through beam positioning and a noise subtraction readout scheme.

Findings

Potential 100-fold reduction in bottom attachment noise at 35-100 Hz.

Approximate 100-fold suppression of top attachment noise with the proposed readout.

Applicable to initial and enhanced LIGO configurations.

Abstract

The suspension noise in interferometric gravitational wave detectors is caused by losses at the top and the bottom attachments of each suspension fiber. We use the Fluctuation-Dissipation theorem to argue that by careful positioning of the laser beam spot on the mirror face it is possible to reduce the contribution of the bottom attachment point to the suspension noise by several orders of magnitude. For example, for the initial and enhanced LIGO design parameters (i.e. mirror masses and sizes, and suspension fibers' lengths and diameters) we predict a reduction of $\sim 100$ in the "bottom" spectral density throughout the band $35-100\hbox{Hz}$ of serious thermal noise. We then propose a readout scheme which suppresses the suspension noise contribution of the top attachment point. The idea is to monitor an averaged horizontal displacement of the fiber of length $ l$; this allows one to record the contribution of the top attachment point to the suspension noise, and later subtract it it from the interferometer readout. For enhanced LIGO this would allow a suppression factor about 100 in spectral density of suspension thermal noise.