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

The suspension noise in interferometric gravitational wave detectors is cau sed by losses at the top and the bottom attachments of each suspension fibr e. We use the fluctuation-dissipation theorem to argue that by careful posi tioning 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 (Laser Interferometer Gravitational Wave Observatory) design parameters (i .e. mirror masses and sizes, and suspension fibres' lengths and diameters) we predict a reduction of similar to 100 in the 'bottom' spectral density t hroughout the band 35-100 Hz of serious thermal noise. We then propose a readout scheme which suppresses the suspension noise cont ribution of the top attachment point. The idea is to monitor an averaged ho rizontal displacement of the fibre of length l; this allows one to record t he contribution of the top attachment point to the suspension noise, and la ter subtract it from the interferometer readout. This method will allow a s uppression factor in spectral density of 7.4(l/d(2))root Mg/pi E, where d i s the fibre's diameter, E is it's Young modulus and M is the mass of the mi rror. For the test mass parameters of the initial and enhanced LIGO designs this reduction factor is 132 x (l/30 cm)(0.6 mm/d)(2). We offer what we think might become a practical implementation of such a re adout scheme. We propose to position a thin optical waveguide close to a fu sed silica fibre used as the suspension fibre. The waveguide itself is at t he surface of a solid fused silica slab which is attached rigidly to the la st mass of the seismic isolation stack (see figure 5). The thermal motion o f the suspension fibre is recorded through the phaseshift of an optical wav e passed through the waveguide. A laser power of 1 mW should be sufficient to achieve the desired sensitivity.