How to reduce suspension thermal noise in LIGO without improving theQof the pendulum and violin modes

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 ∼ 100 in the "bottom" spectral density throughout the band 35 − 100Hz 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 from the interferometer readout. This method will allow a suppression factor in spectral density of 7.4 (l/d 2 ) M g/πE, where d is the fiber's diameter, E is it's Young modulus and M is the mass of the mirror. For the test mass parameters of the initial and enhanced LIGO designs this reduction factor is 132 × (l/30cm)(0.6mm/d) 2 .We offer what we think might become a practical implementation of such a readout scheme. We propose to position a thin optical waveguide close to a fused silica fiber used as the suspension fiber. The waveguide itself is at the surface of a solid fused silica slab which is attached rigidly to the last mass of the seismic isolation stack (see Fig. 5). The thermal motion of the suspension fiber is recorded through the phaseshift of an optical wave passed through the waveguide. A laser power of 1mW should be sufficient to achieve the desired sensitivity.

Thermodynamical fluctuations and photo-thermal shot noise in gravitational wave antennae

“…It is important to emphasize that contrastingly to the gravitational wave, both effects do not displace the centers of masses of the mirrors. Thus, in principle it is possible to subtract these effects as well as usual Brownian fluctuations (with certain level of accuracy) in a way similar to the procedure suggested for the subtraction of thermal noise in suspension fibers [20]. However, till now to our knowledge nobody presented the scheme of such subtraction.…”

Analysis of parametric oscillatory instability in Fabry–Perot cavity with Gauss and Laguerre–Gauss main mode profile

“…Therefore, the numerical analysis of the mode structure should evidently include the case when Z = 0. Note that there is a proposal to use special shift Z of the laser beam of about several centimeters from the mirror axis in order to decrease thermal suspension noise [28].…”

Towards thermal noise free optomechanics

“…The losses into the suspension can be reduced by centre of percussion tuning of the optical spring as demonstrated by Braginsky et al in 1998 [30]. They demonstrated that a reduction factor of 100 could be achieved.…”