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Improving the electronics of the gravitational-wave readout for Advanced LIGO

Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) detectors are extremely complex instruments with sensitivities reaching as low as 2×10-20 m/sqrt(Hz) during operation. To accomplish better astrophysical sensitivity, the instruments are continuously upgraded, applying state-of-the-art technologies during break-time in the year-long observing runs.

The quantised nature of light poses one of the most significant barriers on the improvement of LIGO's sensitivity, with increased power and squeezed states of light being part of the LIGO A+ upgrade (a detector upgrade planned after the next observing run, O4) to reduce the shot noise and radiation pressure present within the system. However, decreasing shot noise with squeezing increases the relevance of other noise sources, specifically that of the electronic noise of the main photodetector readout. To tackle this and allow the use of higher levels of squeezing, a new high-power and low-noise preamplifier circuit for the photodiode readouts has been developed. These preamplifiers are the very first step to extract the gravitational wave signals from the photodiodes. When gravitational waves pass the detector, the photodiode in transmission of the output mode cleaner senses the slight power change of the laser light. The preamplifier then converts the photocurrent signals of a photodiode to a voltage and amplifies those signals.

The new readouts have been developed with the aim of being installed at Hanford and Livingston in time for the 2022 observing run. The final design shows noise level reductions up to a factor of 6 compared to the current readout (Fig. 1) which will increase the amount of observed squeezing upon installation and thereby increase the sensitivity of the LIGO detector to astronomical mergers further out into the universe.

Figure 1: Input referred noise of the new pre-amplifier stage (red) compared to the present noise in Advanced LIGO (black).
Figure 1: Input referred noise of the new pre-amplifier stage (red) compared to the present noise in Advanced LIGO (black).