Abstract
This paper describes the design and performance of an extremely low-noise differential transimpedance amplifier, which takes its two inputs from separate photodiodes. The amplifier was planned to serve as the front-end electronics for a highly sensitive shadow-displacement sensing system, aimed at detecting very low-level "Violin-Mode" (VM) oscillations in 0.4 mm diameter by 600 mm long fused-silica suspension fibres. Four such highly tensioned fibres support the 40 kg test-masses/mirrors of the Advanced Laser Interferometer Gravitational wave Observatory interferometers. This novel design of amplifier incorporates features which prevent "noise-gain peaking" arising from large area photodiode (and cable) capacitances, and which also usefully separate the DC and AC photocurrents coming from the photodiodes. In consequence, the differential amplifier was able to generate straightforwardly two DC outputs, one per photodiode, as well as a single high-gain output for monitoring the VM oscillations-this output being derived from the difference of the photodiodes' two, naturally anti-phase, AC photocurrents. Following a displacement calibration, the amplifier's final VM signal output was found to have an AC displacement responsivity at 500 Hz of (9.43 ± 1.20) MV(rms) m(-1)(rms), and, therefore, a shot-noise limited sensitivity to such AC shadow- (i.e., fibre-) displacements of (69 ± 13) picometres/√Hz at this frequency, over a measuring span of ±0.1 mm.
Highlights
A prototype system of four shadow-sensors was designed to be retro-fitted into an Advanced LIGO (Laser Interferometer Gravitational wave Observatory) test-mass/mirror suspension, in which a 40 kg test-mass was suspended by four fused silica fibres, the dimensions of each fibre being approximately 600 mm long by 0.4 mm in diameter.[1,2,3,4,5,6]
The emitter provided a collimated beam of illumination from a Near InfraRed (NIR: λ = 890 nm) multi-LED source,[7] and this cast a vertical shadow of the illuminated fibre onto its facing, photodiode-based, detector
In sum: the non-inverting integrator feedback forces the DC and “Violin-Mode” AC photocurrents to follow the different paths indicated in the figure
Summary
A prototype system of four shadow-sensors was designed to be retro-fitted into an Advanced LIGO (Laser Interferometer Gravitational wave Observatory) test-mass/mirror suspension, in which a 40 kg test-mass was suspended by four fused silica fibres, the dimensions of each fibre being approximately 600 mm long by 0.4 mm in diameter.[1,2,3,4,5,6] These shadow-sensors—one per suspension fibre—each comprised an optical emitter and detector, bracketing the illuminated fibre. A transimpedance (photocurrent-to-voltage) amplifier was researched for use as the low-noise “front-end” electronics to a single photodiode-based shadow-sensor, with the shadow of the illuminated fibre falling over one vertically orientated edge of the rectangular sensor. In this way, a lateral vibration, or simple displacement, of the silica fibre’s shadow altered the photocurrent flowing through the photodiode (PD). Beneficial, side-effect of this combination of differential detector and amplifier was that proper shadow-alignment with the detector could be carried out more straightforwardly—by virtue of the natural pendulum-mode motion of the monitored fibre and its suspended test-mass It was appreciated that this range could be reduced significantly, with other attendant benefits as described below, if the DC and 0.6 Hz photocurrents were handled separately from the VM signals
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