LIGO Document G0900588-v1
- Due to seismic, or other, forms of mechanical excitation, so-called ‘violin-mode’ resonances can be generated in the silica
fibres of GW interferometer mirror suspensions. The Q-factor of these resonances can be very high, and they need to be
sensed, and damped, because they lie within the working band of the detectors. We have designed and built a shadow-sensor,
potentially for use in Advanced LIGO, with the aim of detecting ‘violin-mode’ resonances of the silica suspension fibres at the 10-10 m level. The
sensor employs a split-photodiode detector measuring 2.2 mm x 29.1 mm, with essentially zero dead-band between the two detectors.
The emitter consists of a vertical column of 16 infrared LEDs (890 nm), which casts a shadow of each fibre onto its associated detector, via
the intermediary of a ‘reverse Galilean’ optical system. Four additional columns of LEDs can be activated, one at a time, in order to steer the shadow
over the centre of the detector—should there be any slow mechanical relaxation of the suspension system over time. In principle this approach can
compensate for a mechanical drift, or fibre-offset, of up to ±0.5 mm. On the detection side a low-noise differential amplifier employs local DC feedback,
so as to maximise the AC dynamic range. Also, local AC feedback across each photodiode has been used so as to prevent any measurable
peaking of the noise-gain, with a capacitance of 680 pF in parallel with the detector (the capacitance mimicking that of a 6 m long cable running to the photodiode).
Preliminary testing of the prototype system has indicated that the desired violin-mode detection sensitivity will be reached, or exceeded.
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