Abstract
The search for gravitational wave signals from astrophysical sources has led to the current work to upgrade the two largest of the long-baseline laser interferometers, the LIGO detectors. The first fused silica mirror suspensions for the Advanced LIGO gravitational wave detectors have been installed at the LIGO Hanford and Livingston sites. These quadruple pendulums use synthetic fused silica fibers produced using a CO2 laser pulling machine to reduce thermal noise in the final suspension stage. The suspension thermal noise in Advanced LIGO is predicted to be limited by internal damping in the surface layer of the fibers, damping in the weld regions, and the strength of the fibers. We present here a new method for increasing the fracture strength of fused silica fibers by laser polishing of the stock material from which they are produced. We also show measurements of mechanical loss in laser polished fibers, showing a reduction of 30% in internal damping in the surface layer.
Highlights
The sensitivity of the coming generation of gravitational wave detectors currently being installed will in part be limited at frequencies between 10 and 100 Hz by Brownian motion of the mirrors and their suspensions [1]
We discuss here the effects which limit the performance of fused silica fibers as a suspension material and will show improvements that can be achieved using a new technique of laser polishing the stock material from which the fiber is pulled
The fibers that are being installed in the Advanced LIGO gravitational wave detector suspensions have a dumbbell shape [29, 30], where the diameter of the ends of the fibers is set to cancel the thermoelastic losses
Summary
The sensitivity of the coming generation of gravitational wave detectors currently being installed will in part be limited at frequencies between 10 and 100 Hz by Brownian motion of the mirrors and their suspensions [1]. The ears have shaped ‘horns’ to which the fibers are welded This non-ideal transition from thin fiber to test mass results in a reduction in the dilution effect and an increase in suspension thermal noise. The vertical bounce mode frequency is set by the cross-sectional area and length of the fused silica fibers in the final stage of the suspension. The fibers that are being installed in the Advanced LIGO gravitational wave detector suspensions have a dumbbell shape [29, 30], where the diameter of the ends of the fibers is set to cancel the thermoelastic losses. To further reduce the horizontal component of Brownian noise at room temperature in a fused silica suspension there are two remaining possibilities: increase the length of the suspension or decrease the dissipation of the surface layer. Measurements of the mechanical dissipation in these fibers are given in this paper, and demonstrate a small but measurable reduction in the internal damping of the surface layer as a result of laser polishing
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