A source of illumination for low-noise ‘Violin-Mode’ shadow sensors, intended for use in interferometric gravitational wave detectors

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Abstract:: A low-noise source of illumination is described for shadow-sensors having 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. These sensors were designed to detect ‘Violin-Mode’ resonances in the suspension fibres of the test-masses/mirrors for the advanced LIGO (Laser Interferometer Gravitational wave Observatory) gravitational wave detectors. The source of illumination (Emitter) described here used a single column of 8 x miniature Near InfraRed LEDs (lambda = 890 nm). These Emitters cast the shadows of 400 um diameter fused silica suspension fibres onto their complementary shadow-displacement detectors, located at a distance of 74 fibre diameters (29.6 mm) from the axes of the fibres themselves. Violin-Mode vibrations of each fibre were sensed as differential AC photocurrents in the corresponding ‘split-photodiode’ detector. This paper describes the measures that were taken in the design of the Emitters, in order to produce high-contrast shadows at such distant detectors with, additionally, transverse vibrational noise at a level close to the fundamental shot noise limit, and the highest possible sensitivity to lateral vibration of the fibres, within the constraints of this simple design of shadow-sensor. The shadow detector is described in an accompanying paper.

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Notes and Changes:: Updated for submission to Measurement Science and Technology

LIGO Document P1100047-v3