LIGO Document P1100046-v3

A ‘Violin-Mode’ shadow sensor for interferometric gravitational wave detectors.

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This paper describes the design of a system of four novel shadow detectors having, collectively, a displacement sensitivity of (6.9 ± 1.3)e-11 m (rms)/sqrt-Hz, at 500 Hz, over a measuring span of ±0.1 mm. The detectors were designed to detect so-called ‘Violin-Mode’ (VM) Eigenmode resonances in the 600 mm long by 400 um diameter silica suspension fibres of the test-masses/mirrors for the advanced LIGO (Laser Interferometer Gravitational wave Observatory) gravitational wave detectors. These modes may be excited by earthquake or mechanical relaxation events, and the fibres then can transfer vibrational energy to the suspended test masses within the gravitational wave detection bandwidth. The fibres’ ring-down time under vacuum, i.e., the consequential interferometer dead-time, is measured in days, as the VM resonances slowly decay away, and consequently these modes must be detected and then (cold-) damped. The VM detection detectors described here had a target sensitivity of 1e-10 m (rms)/sqrt-Hz at 500 Hz, together with, ultimately, a required detection span of +/-0.1 mm about the mean position of each fibre—in order to compensate for potential slow drift over time of fibre position, due to mechanical relaxation. The full sensor system, comprising emitters (sources of illumination) and shadow detectors, therefore met these specifications, and, using these sensors, VM resonances having amplitudes of 1.2 nm, rms, were detected in the suspension fibres of an advanced LIGO dummy test-mass. This paper focuses mainly on the detector side of the shadow-sensors. The emitters are described in an accompanying paper.
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Updated for submission to Measurement Science and Technology
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For publication in Classical and Quantum Gravity.

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